SDL_getenv.c: fix dynamic loading of environ symbol on FreeBSD

The current implementation uses the returned address of the `dlsym` function
directly to load the `environ` symbol. But this function doesn't return the
address to the symbol itself, instead it returns the address to the location
where the actual address is stored, i.e. it's an additional indirection.
Consequently, the implementation fails to load and process the environment
variables successfully.

One example where this error shows up is in the `Dialog API`: in an `X11`
environment, the `zenity` driver requires access to the user's `DISPLAY` and
`XAUTHORITY` environment variables. Because these variables aren't transfered
to the `zenity` process, no dialogs are shown. This can be exercised in the
`test/testdialog.c` testprogram.

The fix changes the indirection level of the `dlsym` call from `char **` to
`char ***`, does a `NULL`-check in case the call failed, and returns the
dereferenced actual adress to the `environ` symbol.

(cherry picked from commit 10458f2cac)

src/stdlib/SDL_getenv.c

@@ -41,7 +41,12 @@
 #define environ (*_NSGetEnviron())
 #elif defined(SDL_PLATFORM_FREEBSD)
 #include <dlfcn.h>
-#define environ ((char **)dlsym(RTLD_DEFAULT, "environ"))
+static char **get_environ_rtld(void)
+{
+    char ***environ_rtld = (char ***)dlsym(RTLD_DEFAULT, "environ");
+    return environ_rtld ? *environ_rtld : NULL;
+}
+#define environ (get_environ_rtld())
 #else
 extern char **environ;
 #endif