avutil/hmac: use av_freep() to avoid leaving stale pointers in memory

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/hmac.c

@@ -127,7 +127,7 @@ void av_hmac_free(AVHMAC *c)
 {
     if (!c)
         return;
-    av_free(c->hash);
+    av_freep(&c->hash);
     av_free(c);
 }