Assert cache->usage never becomes negative

Assert cache->usage never becomes negative as was occurring before the
fix in r52149.

Just in time, the fix in r52149 was made by Ulisses, not me.



SVN revision: 52190

legacy/evas/src/lib/cache/evas_cache_image.c

@@ -1447,6 +1447,7 @@ EAPI int
 evas_cache_image_flush(Evas_Cache_Image *cache)
 {
    assert(cache);
+   assert(cache->usage >= 0);
 
    if (cache->limit == -1) return -1;