Compile warnings in ref_mvs.c

 0:12.77 dav1d/src/ref_mvs.c:1987:21: warning: code will never be executed [-Wunreachable-code]
 0:12.77         MV_REF *l = bx4 ? &cm->cur_frame.mvs[by4*stride+bx4-1] : NULL;
 0:12.77                     ^~~
 0:12.77 dav1d/src/ref_mvs.c:1983:9: note: silence by adding parentheses to mark code as explicitly dead
 0:12.77     if (0 && bx4 == 38 && by4 == 15 && cm->frame_offset == 3 &&
 0:12.77         ^
 0:12.77         /* DISABLES CODE */ ( )
 0:12.77 1 warning generated.