fix mp4 stts box bug. #3609

Job	Run time
utest-regression-blackbox-test	9m 47s
multiple-arch-aarch64	22m 39s
multiple-arch-armv7	22m 0s
cygwin64-cache	1m 44s
cygwin64	15m 2s
fast	1s
build-centos7	4m 58s
build-ubuntu16	3m 6s
build-cross-aarch64	5m 17s
build-cross-arm	5m 20s
build-ubuntu18	3m 6s
build-ubuntu20	3m 33s
coverage	3m 40s
multiple-arch-amd64	2m 31s
test-done	1s
	1h 42m 45s

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