Fixed crash if SDL_SYS_CreateThread() fails (thanks @capehill!)

Fixes https://github.com/libsdl-org/SDL/issues/15340

src/thread/SDL_thread.c

@@ -411,7 +411,7 @@ SDL_Thread *SDL_CreateThreadWithPropertiesRuntime(SDL_PropertiesID props,
         SDL_DestroySemaphore(thread->ready_sem);
         SDL_free(thread->name);
         SDL_free(thread);
-        thread = NULL;
+		return NULL;
     }
 
     SDL_WaitSemaphore(thread->ready_sem);