avcodec/mpegvideo_enc: Don't apply non-zero offset to null pointer

Affected many FATE tests (mostly vsynth ones).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit 4863671d88)

libavcodec/mpegvideo_enc.c

@@ -1696,7 +1696,8 @@ no_output_pic:
             // input is not a shared pix -> reuse buffer for current_pix
             s->current_picture_ptr = s->reordered_input_picture[0];
             for (i = 0; i < 4; i++) {
-                s->new_picture.f->data[i] += INPLACE_OFFSET;
+                if (s->new_picture.f->data[i])
+                    s->new_picture.f->data[i] += INPLACE_OFFSET;
             }
         }
         ff_mpeg_unref_picture(s->avctx, &s->current_picture);