Fixed crash if initialization of EGL failed but was tried again later.

The internal function SDL_EGL_LoadLibrary() did not delete and remove a mostly
uninitialized data structure if loading the library first failed. A later try to
use EGL then skipped initialization and assumed it was previously successful
because the data structure now already existed. This led to at least one crash
in the internal function SDL_EGL_ChooseConfig() because a NULL pointer was
dereferenced to make a call to eglBindAPI().

src/thread/generic/SDL_sysmutex.c

@@ -0,0 +1,165 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2015 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+#include "../../SDL_internal.h"
+
+/* An implementation of mutexes using semaphores */
+
+#include "SDL_thread.h"
+#include "SDL_systhread_c.h"
+
+
+struct SDL_mutex
+{
+    int recursive;
+    SDL_threadID owner;
+    SDL_sem *sem;
+};
+
+/* Create a mutex */
+SDL_mutex *
+SDL_CreateMutex(void)
+{
+    SDL_mutex *mutex;
+
+    /* Allocate mutex memory */
+    mutex = (SDL_mutex *) SDL_malloc(sizeof(*mutex));
+    if (mutex) {
+        /* Create the mutex semaphore, with initial value 1 */
+        mutex->sem = SDL_CreateSemaphore(1);
+        mutex->recursive = 0;
+        mutex->owner = 0;
+        if (!mutex->sem) {
+            SDL_free(mutex);
+            mutex = NULL;
+        }
+    } else {
+        SDL_OutOfMemory();
+    }
+    return mutex;
+}
+
+/* Free the mutex */
+void
+SDL_DestroyMutex(SDL_mutex * mutex)
+{
+    if (mutex) {
+        if (mutex->sem) {
+            SDL_DestroySemaphore(mutex->sem);
+        }
+        SDL_free(mutex);
+    }
+}
+
+/* Lock the mutex */
+int
+SDL_LockMutex(SDL_mutex * mutex)
+{
+#if SDL_THREADS_DISABLED
+    return 0;
+#else
+    SDL_threadID this_thread;
+
+    if (mutex == NULL) {
+        return SDL_SetError("Passed a NULL mutex");
+    }
+
+    this_thread = SDL_ThreadID();
+    if (mutex->owner == this_thread) {
+        ++mutex->recursive;
+    } else {
+        /* The order of operations is important.
+           We set the locking thread id after we obtain the lock
+           so unlocks from other threads will fail.
+         */
+        SDL_SemWait(mutex->sem);
+        mutex->owner = this_thread;
+        mutex->recursive = 0;
+    }
+
+    return 0;
+#endif /* SDL_THREADS_DISABLED */
+}
+
+/* try Lock the mutex */
+int
+SDL_TryLockMutex(SDL_mutex * mutex)
+{
+#if SDL_THREADS_DISABLED
+    return 0;
+#else
+    int retval = 0;
+    SDL_threadID this_thread;
+
+    if (mutex == NULL) {
+        return SDL_SetError("Passed a NULL mutex");
+    }
+
+    this_thread = SDL_ThreadID();
+    if (mutex->owner == this_thread) {
+        ++mutex->recursive;
+    } else {
+        /* The order of operations is important.
+         We set the locking thread id after we obtain the lock
+         so unlocks from other threads will fail.
+         */
+        retval = SDL_SemWait(mutex->sem);
+        if (retval == 0) {
+            mutex->owner = this_thread;
+            mutex->recursive = 0;
+        }
+    }
+
+    return retval;
+#endif /* SDL_THREADS_DISABLED */
+}
+
+/* Unlock the mutex */
+int
+SDL_mutexV(SDL_mutex * mutex)
+{
+#if SDL_THREADS_DISABLED
+    return 0;
+#else
+    if (mutex == NULL) {
+        return SDL_SetError("Passed a NULL mutex");
+    }
+
+    /* If we don't own the mutex, we can't unlock it */
+    if (SDL_ThreadID() != mutex->owner) {
+        return SDL_SetError("mutex not owned by this thread");
+    }
+
+    if (mutex->recursive) {
+        --mutex->recursive;
+    } else {
+        /* The order of operations is important.
+           First reset the owner so another thread doesn't lock
+           the mutex and set the ownership before we reset it,
+           then release the lock semaphore.
+         */
+        mutex->owner = 0;
+        SDL_SemPost(mutex->sem);
+    }
+    return 0;
+#endif /* SDL_THREADS_DISABLED */
+}
+
+/* vi: set ts=4 sw=4 expandtab: */