avcodec/mpegvideo_dec: Add NULL pointer checks to MPV_motion_lowres()

This makes the null pointer checks match mpv_motion_internal()

Fixes: NULL pointer dereference
Fixes: 59671/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPEG1VIDEO_fuzzer-4993004566609920
Fixes: 59678/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPEGVIDEO_fuzzer-4893168991338496

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpegvideo_dec.c

@@ -885,10 +885,9 @@ static inline void MPV_motion_lowres(MpegEncContext *s,
                                s->mv[dir][1][0], s->mv[dir][1][1],
                                block_s, mb_y);
         } else {
-            if (s->picture_structure != s->field_select[dir][0] + 1 &&
-                s->pict_type != AV_PICTURE_TYPE_B && !s->first_field) {
+            if (   s->picture_structure != s->field_select[dir][0] + 1 && s->pict_type != AV_PICTURE_TYPE_B && !s->first_field
+                || !ref_picture[0]) {
                 ref_picture = s->current_picture_ptr->f->data;
-
             }
             mpeg_motion_lowres(s, dest_y, dest_cb, dest_cr,
                                0, 0, s->field_select[dir][0],
@@ -901,8 +900,9 @@ static inline void MPV_motion_lowres(MpegEncContext *s,
         for (int i = 0; i < 2; i++) {
             uint8_t *const *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;
@@ -933,6 +933,9 @@ static inline void MPV_motion_lowres(MpegEncContext *s,
                 pix_op = s->h264chroma.avg_h264_chroma_pixels_tab;
             }
         } else {
+            if (!ref_picture[0]) {
+                ref_picture = s->current_picture_ptr->f->data;
+            }
             for (int i = 0; i < 2; i++) {
                 mpeg_motion_lowres(s, dest_y, dest_cb, dest_cr,
                                    0, 0, s->picture_structure != i + 1,