avcodec/movtextenc: Reset array counter after freeing array

Otherwise the mov_text encoder can segfault when given subtitles with more
than one AVSubtitleRect if one of the first nb_rects - 1 rects contained
a style attribute.

Reviewed-by: Philip Langdale <philipl@overt.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit 8d4431955c)

libavcodec/movtextenc.c

@@ -102,6 +102,7 @@ static void mov_text_cleanup(MovTextContext *s)
             av_freep(&s->style_attributes[j]);
         }
         av_freep(&s->style_attributes);
+        s->count = 0;
     }
     if (s->style_attributes_temp) {
         *s->style_attributes_temp = s->d;