to_lavc_vid_conv: extra padding for alloc

fixes `uv -t testcard:codec=R12L:size=1921x1080 -c lavc`

src/libavcodec/to_lavc_vid_conv.c

@@ -1756,7 +1756,9 @@ struct to_lavc_vid_conv *to_lavc_vid_conv_init(codec_t in_pixfmt, int width, int
                         av_get_pix_fmt_name(out_pixfmt), get_codec_name(s->decoded_codec));
         watch_pixfmt_degrade(MOD_NAME, get_pixfmt_desc(in_pixfmt), get_pixfmt_desc(s->decoded_codec));
         watch_pixfmt_degrade(MOD_NAME, get_pixfmt_desc(s->decoded_codec), av_pixfmt_get_desc(out_pixfmt));
-        s->decoded = (unsigned char *) malloc((long) vc_get_linesize(width, s->decoded_codec) * height);
+        s->decoded = (unsigned char *) malloc(
+            (long) vc_get_linesize(width, s->decoded_codec) * height +
+            MAX_PADDING);
 
         s->pixfmt_conv_callback = select_pixfmt_callback(out_pixfmt, s->decoded_codec);