lavc: use av_mallocz to allocate AVFrames.

Otherwise the frame is uninitialized, so avcodec_get_frame_defaults()
cannot determine whether to free extended_data.

libavcodec/utils.c

@@ -645,7 +645,7 @@ void avcodec_get_frame_defaults(AVFrame *frame)
 
 AVFrame *avcodec_alloc_frame(void)
 {
-    AVFrame *frame = av_malloc(sizeof(AVFrame));
+    AVFrame *frame = av_mallocz(sizeof(AVFrame));
 
     if (frame == NULL)
         return NULL;