os2: SDL_DestroyMutex should ignore NULL mutexes.

Every other backend does this, so this should match, now.
It's possible this was harmless, but we can avoid the system call
and the (likely?) debug message when it fails, though!

src/thread/os2/SDL_sysmutex.c

@@ -56,12 +56,12 @@ SDL_CreateMutex(void)
 void
 SDL_DestroyMutex(SDL_mutex * mutex)
 {
-    ULONG ulRC;
     HMTX  hMtx = (HMTX)mutex;
-
-    ulRC = DosCloseMutexSem(hMtx);
-    if (ulRC != NO_ERROR) {
-        debug_os2("DosCloseMutexSem(), rc = %u", ulRC);
+    if (hMtx != NULLHANDLE) {
+        const ULONG ulRC = DosCloseMutexSem(hMtx);
+        if (ulRC != NO_ERROR) {
+            debug_os2("DosCloseMutexSem(), rc = %u", ulRC);
+        }
     }
 }