ivi_common: check ref_tile size, fix out of array accesses

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/ivi_common.c

@@ -308,7 +308,10 @@ av_cold int ff_ivi_init_tiles(IVIPlaneDesc *planes, int tile_width, int tile_hei
 
                     tile->ref_mbs = 0;
                     if (p || b) {
+                        if (tile->num_MBs <= ref_tile->num_MBs) {
                             tile->ref_mbs = ref_tile->mbs;
+                        }else
+                            av_log(0, AV_LOG_DEBUG, "Cannot use ref_tile, too few mbs\n");
                         ref_tile++;
                     }