Pointer as bool (libsdl-org#7214)

src/atomic/SDL_spinlock.c

@@ -63,7 +63,7 @@ SDL_bool SDL_AtomicTryLock(SDL_SpinLock *lock)
     /* Terrible terrible damage */
     static SDL_Mutex *_spinlock_mutex;
 
-    if (_spinlock_mutex == NULL) {
+    if (!_spinlock_mutex) {
         /* Race condition on first lock... */
         _spinlock_mutex = SDL_CreateMutex();
     }