Fix crash

[mihail812]

Testing.tRunner.func1(0xc0000f8400)
/home/travis/.gimme/versions/go1.11.5.linux.amd64/src/testing/testing.go:792 +0x387
panic(0x69a900, 0x8fd240)
/home/travis/.gimme/versions/go1.11.5.linux.amd64/src/runtime/panic.go:513 +0x1b9
github.com/VKCOM/joy4/codec/h264parser.SplitNALUs(0xc000018808, 0x4, 0x8, 0x0, 0x0, 0x0, 0x0)
/home/travis/gopath/src/github.com/VKCOM/joy4/codec/h264parser/parser.go:230 +0x6a9
github.com/VKCOM/joy4/codec/h264parser.TestParserNoCrash(0xc0000f8400)
/home/travis/gopath/src/github.com/VKCOM/joy4/codec/h264parser/parser_test.go:48 +0xca
testing.tRunner(0xc0000f8400, 0x6ff238)
/home/travis/.gimme/versions/go1.11.5.linux.amd64/src/testing/testing.go:827 +0xbf
created by testing.(*T).Run
/home/travis/.gimme/versions/go1.11.5.linux.amd64/src/testing/testing.go:878 +0x35c