fix debug build after 16ec1057

Remove debug code under '#if !defined(NDEBUG)' still using av_clamp_mv().
8 jobs for master in 2 minutes and 23 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build-debian #5276
debian amd64

00:00:30

passed build-debian-aarch64 #5280
aarch64 debian

00:01:41

passed build-debian-aarch64-clang-5 #5281
aarch64 clang5 debian

00:01:18

passed build-debian-static #5277
debian amd64

00:00:30

passed build-debian-werror #5283
aarch64 debian

00:01:02

passed build-macos #5282
macos

00:00:23

passed build-win32 #5278
win32

00:00:28

passed build-win64 #5279
win64

00:00:32