picture: assert that there is a destroy callback

Pictures without destroy callback are allocated on the stack and never
explicitly released.

src/misc/picture.c

@@ -289,7 +289,7 @@ void picture_Release( picture_t *p_picture )
         return;
 
     PictureDestroyContext( p_picture );
-    if( p_picture->gc.pf_destroy != NULL )
+    assert( p_picture->gc.pf_destroy != NULL );
     p_picture->gc.pf_destroy( p_picture );
 }