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/windows/SDL_syssem.c

@@ -0,0 +1,148 @@
+/*
+  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"
+
+#if SDL_THREAD_WINDOWS
+
+/* Semaphore functions using the Win32 API */
+
+#include "../../core/windows/SDL_windows.h"
+
+#include "SDL_thread.h"
+
+struct SDL_semaphore
+{
+    HANDLE id;
+    LONG count;
+};
+
+
+/* Create a semaphore */
+SDL_sem *
+SDL_CreateSemaphore(Uint32 initial_value)
+{
+    SDL_sem *sem;
+
+    /* Allocate sem memory */
+    sem = (SDL_sem *) SDL_malloc(sizeof(*sem));
+    if (sem) {
+        /* Create the semaphore, with max value 32K */
+        sem->id = CreateSemaphore(NULL, initial_value, 32 * 1024, NULL);
+        sem->count = initial_value;
+        if (!sem->id) {
+            SDL_SetError("Couldn't create semaphore");
+            SDL_free(sem);
+            sem = NULL;
+        }
+    } else {
+        SDL_OutOfMemory();
+    }
+    return (sem);
+}
+
+/* Free the semaphore */
+void
+SDL_DestroySemaphore(SDL_sem * sem)
+{
+    if (sem) {
+        if (sem->id) {
+            CloseHandle(sem->id);
+            sem->id = 0;
+        }
+        SDL_free(sem);
+    }
+}
+
+int
+SDL_SemWaitTimeout(SDL_sem * sem, Uint32 timeout)
+{
+    int retval;
+    DWORD dwMilliseconds;
+
+    if (!sem) {
+        return SDL_SetError("Passed a NULL sem");
+    }
+
+    if (timeout == SDL_MUTEX_MAXWAIT) {
+        dwMilliseconds = INFINITE;
+    } else {
+        dwMilliseconds = (DWORD) timeout;
+    }
+    switch (WaitForSingleObject(sem->id, dwMilliseconds)) {
+    case WAIT_OBJECT_0:
+        InterlockedDecrement(&sem->count);
+        retval = 0;
+        break;
+    case WAIT_TIMEOUT:
+        retval = SDL_MUTEX_TIMEDOUT;
+        break;
+    default:
+        retval = SDL_SetError("WaitForSingleObject() failed");
+        break;
+    }
+    return retval;
+}
+
+int
+SDL_SemTryWait(SDL_sem * sem)
+{
+    return SDL_SemWaitTimeout(sem, 0);
+}
+
+int
+SDL_SemWait(SDL_sem * sem)
+{
+    return SDL_SemWaitTimeout(sem, SDL_MUTEX_MAXWAIT);
+}
+
+/* Returns the current count of the semaphore */
+Uint32
+SDL_SemValue(SDL_sem * sem)
+{
+    if (!sem) {
+        SDL_SetError("Passed a NULL sem");
+        return 0;
+    }
+    return (Uint32)sem->count;
+}
+
+int
+SDL_SemPost(SDL_sem * sem)
+{
+    if (!sem) {
+        return SDL_SetError("Passed a NULL sem");
+    }
+    /* Increase the counter in the first place, because
+     * after a successful release the semaphore may
+     * immediately get destroyed by another thread which
+     * is waiting for this semaphore.
+     */
+    InterlockedIncrement(&sem->count);
+    if (ReleaseSemaphore(sem->id, 1, NULL) == FALSE) {
+        InterlockedDecrement(&sem->count);      /* restore */
+        return SDL_SetError("ReleaseSemaphore() failed");
+    }
+    return 0;
+}
+
+#endif /* SDL_THREAD_WINDOWS */
+
+/* vi: set ts=4 sw=4 expandtab: */