avcodec/mpegvideo_motion: Use a field from the current frame if the last is...

avcodec/mpegvideo_motion: Use a field from the current frame if the last is unavailable in DMV & 16x8

Fixes null pointer dereference

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

libavcodec/mpegvideo_motion.c

@@ -824,8 +824,8 @@ static av_always_inline void MPV_motion_internal(MpegEncContext *s,
         for(i=0; i<2; i++){
             uint8_t ** ref2picture;
 
-            if(s->picture_structure == s->field_select[dir][i] + 1
-               || s->pict_type == AV_PICTURE_TYPE_B || s->first_field){
+            if((s->picture_structure == s->field_select[dir][i] + 1
+               || s->pict_type == AV_PICTURE_TYPE_B || s->first_field) && ref_picture[0]){
                 ref2picture= ref_picture;
             }else{
                 ref2picture = s->current_picture_ptr->f.data;
@@ -854,6 +854,9 @@ static av_always_inline void MPV_motion_internal(MpegEncContext *s,
                 pix_op = s->hdsp.avg_pixels_tab;
             }
         }else{
+            if (!ref_picture[0]) {
+                ref_picture = s->current_picture_ptr->f.data;
+            }
             for(i=0; i<2; i++){
                 mpeg_motion(s, dest_y, dest_cb, dest_cr,
                             s->picture_structure != i+1,