avcodec/movtextdec: check that ftab has been allocated before dereferencing it

Fixes potential null pointer dereference on deallocation
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/movtextdec.c

@@ -93,10 +93,12 @@ static void mov_text_cleanup(MovTextContext *m)
 static void mov_text_cleanup_ftab(MovTextContext *m)
 {
     int i;
+    if (m->ftab) {
         for(i = 0; i < m->count_f; i++) {
             av_freep(&m->ftab[i]->font);
             av_freep(&m->ftab[i]);
         }
+    }
     av_freep(&m->ftab);
 }