lavc/dvdsubenc: check the type of rectangles.

Avoid a crash if a text rectangle is provided.
Fix the segfault reported in trac ticket #1661.

libavcodec/dvdsubenc.c

@@ -256,6 +256,11 @@ static int encode_dvd_subtitles(AVCodecContext *avctx,
 
     if (rects == 0 || h->rects == NULL)
         return AVERROR(EINVAL);
+    for (i = 0; i < rects; i++)
+        if (h->rects[i]->type != SUBTITLE_BITMAP) {
+            av_log(avctx, AV_LOG_ERROR, "Bitmap subtitle required\n");
+            return AVERROR(EINVAL);
+        }
     vrect = *h->rects[0];
 
     if (rects > 1) {