indeo: check for reference when inheriting mvs

The same is done already for qdelta.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
(cherry picked from commit b36e1893ef)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>

libavcodec/ivi_common.c

@@ -599,7 +599,7 @@ static int ivi_process_empty_tile(AVCodecContext *avctx, IVIBandDesc *band,
             if (band->inherit_qdelta && ref_mb)
                 mb->q_delta = ref_mb->q_delta;
 
-            if (band->inherit_mv) {
+            if (band->inherit_mv && ref_mb) {
                 /* motion vector inheritance */
                 if (mv_scale) {
                     mb->mv_x = ivi_scale_mv(ref_mb->mv_x, mv_scale);