fix mp4 stts box bug. #3610

Job	Run time
cygwin64-cache	1m 36s
multiple-arch-aarch64	22m 56s
multiple-arch-armv7	22m 3s
utest-regression-blackbox-test	9m 42s
fast	1s
cygwin64	15m 24s
build-ubuntu16	3m 5s
build-ubuntu18	3m 9s
build-centos7	5m 6s
build-cross-arm	5m 18s
build-ubuntu20	3m 35s
build-cross-aarch64	5m 10s
multiple-arch-amd64	2m 21s
coverage	3m 36s
test-done	1s
	1h 43m 3s

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