cache: use av_freep instead of av_free in cache_open

Signed-off-by: Jean First <jeanfirst@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/cache.c

@@ -63,7 +63,7 @@ static int cache_open(URLContext *h, const char *arg, int flags)
     }
 
     unlink(buffername);
-    av_free(buffername);
+    av_freep(&buffername);
 
     return ffurl_open(&c->inner, arg, flags, &h->interrupt_callback, NULL);
 }