lavu: Refactor side data wiping

And make sure the nb_side_data field is reset as well.

Based on an initial patch from wm4 <nfxjfg@googlemail.com>.

CC: libav-stable@libav.org

libavutil/frame.c

@@ -54,6 +54,18 @@ static void free_side_data(AVFrameSideData **ptr_sd)
     av_freep(ptr_sd);
 }
 
+static void wipe_side_data(AVFrame *frame)
+{
+    int i;
+
+    for (i = 0; i < frame->nb_side_data; i++) {
+        free_side_data(&frame->side_data[i]);
+    }
+    frame->nb_side_data = 0;
+
+    av_freep(&frame->side_data);
+}
+
 AVFrame *av_frame_alloc(void)
 {
     AVFrame *frame = av_mallocz(sizeof(*frame));
@@ -284,10 +296,7 @@ void av_frame_unref(AVFrame *frame)
 {
     int i;
 
-    for (i = 0; i < frame->nb_side_data; i++) {
-        free_side_data(&frame->side_data[i]);
-    }
-    av_freep(&frame->side_data);
+    wipe_side_data(frame);
 
     for (i = 0; i < FF_ARRAY_ELEMS(frame->buf); i++)
         av_buffer_unref(&frame->buf[i]);
@@ -398,10 +407,7 @@ int av_frame_copy_props(AVFrame *dst, const AVFrame *src)
         AVFrameSideData *sd_dst = av_frame_new_side_data(dst, sd_src->type,
                                                          sd_src->size);
         if (!sd_dst) {
-            for (i = 0; i < dst->nb_side_data; i++) {
-                free_side_data(&dst->side_data[i]);
-            }
-            av_freep(&dst->side_data);
+            wipe_side_data(dst);
             return AVERROR(ENOMEM);
         }
         memcpy(sd_dst->data, sd_src->data, sd_src->size);