lavc: add avcodec_free_frame().

Since an AVFrame now has malloced members (extended_data), it must have
a destructor.

libavcodec/utils.c

@@ -659,6 +659,21 @@ AVFrame *avcodec_alloc_frame(void)
     return frame;
 }
 
+void avcodec_free_frame(AVFrame **frame)
+{
+    AVFrame *f;
+
+    if (!frame || !*frame)
+        return;
+
+    f = *frame;
+
+    if (f->extended_data != f->data)
+        av_freep(&f->extended_data);
+
+    av_freep(frame);
+}
+
 int attribute_align_arg avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options)
 {
     int ret = 0;