crypto: Use av_freep instead of av_free

Using av_freep is generally good practice.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/crypto.c

@@ -153,9 +153,9 @@ static int crypto_close(URLContext *h)
     CryptoContext *c = h->priv_data;
     if (c->hd)
         ffurl_close(c->hd);
-    av_free(c->aes);
-    av_free(c->key);
-    av_free(c->iv);
+    av_freep(&c->aes);
+    av_freep(&c->key);
+    av_freep(&c->iv);
     return 0;
 }