sd_lavc: skip 0 sized sub-bitmaps

Not everything in the OSD path handles 0x0 sized sub-bitmaps well. At
least the code implementing --sub-gray had severe problems with it.

Fix this by skipping such bitmaps.

sub/sd_lavc.c

@@ -183,11 +183,13 @@ static void decode(struct sd *sd, struct demux_packet *packet)
                                       sub.num_rects);
             for (int i = 0; i < sub.num_rects; i++) {
                 struct AVSubtitleRect *r = sub.rects[i];
-                struct sub_bitmap *b = &priv->inbitmaps[i];
-                struct osd_bmp_indexed *img = &priv->imgs[i];
+                struct sub_bitmap *b = &priv->inbitmaps[priv->count];
+                struct osd_bmp_indexed *img = &priv->imgs[priv->count];
                 if (!(r->flags & AV_SUBTITLE_FLAG_FORCED) &&
                     opts->forced_subs_only)
                     continue;
+                if (r->w == 0 || r->h == 0)
+                    continue;
                 img->bitmap = r->pict.data[0];
                 assert(r->nb_colors > 0);
                 assert(r->nb_colors * 4 <= sizeof(img->palette));