indeo4: Do not access missing reference MV

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/indeo4.c

@@ -459,7 +459,7 @@ static int decode_mb_info(IVI45DecContext *ctx, IVIBandDesc *band,
                 }
 
                 mb->mv_x = mb->mv_y = 0; /* no motion vector coded */
-                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);
@@ -471,7 +471,10 @@ static int decode_mb_info(IVI45DecContext *ctx, IVIBandDesc *band,
                 }
             } else {
                 if (band->inherit_mv) {
-                    mb->type = ref_mb->type; /* copy mb_type from corresponding reference mb */
+                    /* copy mb_type from corresponding reference mb */
+                    if (!ref_mb)
+                        return AVERROR_INVALIDDATA;
+                    mb->type = ref_mb->type;
                 } else if (ctx->frame_type == FRAMETYPE_INTRA ||
                            ctx->frame_type == FRAMETYPE_INTRA1) {
                     mb->type = 0; /* mb_type is always INTRA for intra-frames */
@@ -495,6 +498,7 @@ static int decode_mb_info(IVI45DecContext *ctx, IVIBandDesc *band,
                     mb->mv_x = mb->mv_y = 0; /* there is no motion vector in intra-macroblocks */
                 } else {
                     if (band->inherit_mv) {
+                        if (ref_mb)
                             /* motion vector inheritance */
                             if (mv_scale) {
                                 mb->mv_x = ivi_scale_mv(ref_mb->mv_x, mv_scale);