wiiu/thread: Add missing NULL checks to mutex code

According to  the changes in d59caff:
> [...] SDL has the concept of a NULL mutex, so the mutex functions have been changed not to report errors if a mutex hasn't been initialized.
> We do have mutexes that might be accessed when they are NULL, notably in the event system, so this is an important change.

src/thread/wiiu/SDL_sysmutex.c

@@ -34,35 +34,51 @@ SDL_CreateMutex(void)
 
     /* Allocate the structure */
     mutex = (OSMutex *) SDL_calloc(1, sizeof(OSMutex));
-    OSInitMutex(mutex);
+    if (mutex != NULL) {
+        OSInitMutex(mutex);
+    } else {
+        SDL_OutOfMemory();
+    }
     return (SDL_mutex *)mutex;
 }
 
 void
 SDL_DestroyMutex(SDL_mutex * mutex)
 {
-    if (mutex) {
+    if (mutex != NULL) {
         SDL_free(mutex);
     }
 }
 
 /* Lock the mutex */
 int
-SDL_LockMutex(SDL_mutex * mutex)
+SDL_LockMutex(SDL_mutex * mutex) SDL_NO_THREAD_SAFETY_ANALYSIS /* clang doesn't know about NULL mutexes */
 {
+    if (mutex == NULL) {
+        return 0;
+    }
+
     OSLockMutex((OSMutex *)mutex);
     return 0;
 }
 
 int
 SDL_TryLockMutex(SDL_mutex * mutex)
 {
+    if (mutex == NULL) {
+        return 0;
+    }
+
     return OSTryLockMutex((OSMutex *)mutex) ? 0 : SDL_MUTEX_TIMEDOUT;
 }
 
 int
-SDL_UnlockMutex(SDL_mutex * mutex)
+SDL_UnlockMutex(SDL_mutex * mutex) SDL_NO_THREAD_SAFETY_ANALYSIS /* clang doesn't know about NULL mutexes */
 {
+    if (mutex == NULL) {
+        return 0;
+    }
+
     OSUnlockMutex((OSMutex *)mutex);
     return 0;
 }