fix mp4 stts box bug. #3607

Job	Run time
multiple-arch-armv7	22m 1s
cygwin64-cache	1m 48s
multiple-arch-aarch64	22m 34s
utest-regression-blackbox-test	8m 12s
fast	1s
cygwin64	17m 17s
build-ubuntu20	3m 34s
build-centos7	4m 51s
build-ubuntu16	3m 1s
build-cross-aarch64	5m 3s
coverage	3m 34s
multiple-arch-amd64	2m 25s
build-cross-arm	5m 11s
build-ubuntu18	3m 7s
test-done	0s
	1h 42m 39s

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