fix shutdown crash when use OpenGL backend

Author: debugly

ijkmedia/wrapper/apple/FSPlayer.m

@@ -607,7 +607,9 @@ - (void)shutdown
     [_hudTimer invalidate];
     _hudTimer = nil;
     //clean refresh observer
-    [_glView registerRefreshCurrentPicObserver:NULL];
+    if ([_glView respondsToSelector:@selector(registerRefreshCurrentPicObserver:)]) {
+        [_glView registerRefreshCurrentPicObserver:NULL];
+    }
     [self performSelectorInBackground:@selector(shutdownWaitStop:) withObject:self];
 }