fftools/ffmpeg: make sure FrameData is writable when we modify it

Also, add a function that returns const FrameData* for cases that only
read from it.

fftools/ffmpeg.c

@@ -427,21 +427,34 @@ InputStream *ist_iter(InputStream *prev)
     return NULL;
 }
 
-FrameData *frame_data(AVFrame *frame)
+static int frame_data_ensure(AVFrame *frame, int writable)
 {
     if (!frame->opaque_ref) {
         FrameData *fd;
 
         frame->opaque_ref = av_buffer_allocz(sizeof(*fd));
         if (!frame->opaque_ref)
-            return NULL;
+            return AVERROR(ENOMEM);
         fd = (FrameData*)frame->opaque_ref->data;
 
         fd->dec.frame_num = UINT64_MAX;
         fd->dec.pts       = AV_NOPTS_VALUE;
-    }
+    } else if (writable)
+        return av_buffer_make_writable(&frame->opaque_ref);
+
+    return 0;
+}
 
-    return (FrameData*)frame->opaque_ref->data;
+FrameData *frame_data(AVFrame *frame)
+{
+    int ret = frame_data_ensure(frame, 1);
+    return ret < 0 ? NULL : (FrameData*)frame->opaque_ref->data;
+}
+
+const FrameData *frame_data_c(AVFrame *frame)
+{
+    int ret = frame_data_ensure(frame, 0);
+    return ret < 0 ? NULL : (const FrameData*)frame->opaque_ref->data;
 }
 
 void remove_avoptions(AVDictionary **a, AVDictionary *b)

fftools/ffmpeg.h

@@ -726,6 +726,8 @@ int subtitle_wrap_frame(AVFrame *frame, AVSubtitle *subtitle, int copy);
  */
 FrameData *frame_data(AVFrame *frame);
 
+const FrameData *frame_data_c(AVFrame *frame);
+
 int ifilter_send_frame(InputFilter *ifilter, AVFrame *frame, int keep_reference);
 int ifilter_send_eof(InputFilter *ifilter, int64_t pts, AVRational tb);
 int ifilter_sub2video(InputFilter *ifilter, const AVFrame *frame);

fftools/ffmpeg_filter.c

@@ -1859,9 +1859,9 @@ static int choose_out_timebase(OutputFilterPriv *ofp, AVFrame *frame)
     FPSConvContext   *fps = &ofp->fps;
     AVRational        tb = (AVRational){ 0, 0 };
     AVRational fr;
-    FrameData *fd;
+    const FrameData *fd;
 
-    fd = frame_data(frame);
+    fd = frame_data_c(frame);
 
     // apply -enc_time_base
     if (ofp->enc_timebase.num == ENC_TIME_BASE_DEMUX &&