dvbsubdec: Fixed segfault when decoding subtitles

This fixes a segfault (originally found in Movian, but traced to libav)
when decoding subtitles because only an array of rects is allocated,
but not the actual structs it contains. The issue was probably
introduced in commit 23833236 where the loop to allocate the rects in
the array was thrown away.
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>

libavcodec/dvbsubdec.c

@@ -1285,13 +1285,18 @@ static int dvbsub_display_end_segment(AVCodecContext *avctx, const uint8_t *buf,
     }
 
     sub->num_rects = ctx->display_list_size;
-    if (sub->num_rects <= 0)
-        return AVERROR_INVALIDDATA;
 
-    sub->rects = av_mallocz_array(sub->num_rects * sub->num_rects,
-                                  sizeof(*sub->rects));
-    if (!sub->rects)
-        return AVERROR(ENOMEM);
+    if (sub->num_rects > 0) {
+        sub->rects = av_mallocz(sizeof(*sub->rects) * sub->num_rects);
+        if (!sub->rects)
+            return AVERROR(ENOMEM);
+        for (i = 0; i < sub->num_rects; i++) {
+            sub->rects[i] = av_mallocz(sizeof(*sub->rects[i]));
+            if (!sub->rects[i]) {
+                return AVERROR(ENOMEM);
+            }
+        }
+    }
 
     i = 0;