Revert "avutil/frame: Disallow zero sized frame side data"

Found a case where we use size==0, the other related commits
remain needed, and should be sufficient to fix the original issue

This reverts commit 7e4f32f4.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavutil/frame.c

@@ -668,8 +668,6 @@ AVFrameSideData *av_frame_new_side_data(AVFrame *frame,
                                         enum AVFrameSideDataType type,
                                         int size)
 {
-    if (size <= 0)
-        return NULL;
 
     return frame_new_side_data(frame, type, av_buffer_alloc(size));
 }