avcodec/snow: ensure current_picture is writable before modifying its data

current_picture was not writable here because a reference existed in
at least avctx->coded_frame, and potentially elsewhere if the caller
created new ones from it.
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/snowenc.c

@@ -1625,10 +1625,22 @@ static int encode_frame(AVCodecContext *avctx, AVPacket *pkt,
         s->lambda = 0;
     }//else keep previous frame's qlog until after motion estimation
 
+#if FF_API_CODED_FRAME
+FF_DISABLE_DEPRECATION_WARNINGS
+    av_frame_unref(avctx->coded_frame);
+FF_ENABLE_DEPRECATION_WARNINGS
+#endif
+
     if (s->current_picture->data[0]) {
         int w = s->avctx->width;
         int h = s->avctx->height;
 
+#if FF_API_CODED_FRAME
+        ret = av_frame_make_writable(s->current_picture);
+        if (ret < 0)
+            return ret;
+#endif
+
         s->mpvencdsp.draw_edges(s->current_picture->data[0],
                                 s->current_picture->linesize[0], w   , h   ,
                                 EDGE_WIDTH  , EDGE_WIDTH  , EDGE_TOP | EDGE_BOTTOM);
@@ -1646,7 +1658,6 @@ static int encode_frame(AVCodecContext *avctx, AVPacket *pkt,
     ff_snow_frame_start(s);
 #if FF_API_CODED_FRAME
 FF_DISABLE_DEPRECATION_WARNINGS
-    av_frame_unref(avctx->coded_frame);
     ret = av_frame_ref(avctx->coded_frame, s->current_picture);
 FF_ENABLE_DEPRECATION_WARNINGS
 #endif