Avoid left shift of negative values when parsing H264 stream
Avoid left shift of negative values when parsing H264 stream

Emulation prevention three-byte detection only needs the previous
two bytes, so limit |prev_two_bytes_| to be just two bytes.

BUG=603485
TEST=test locally with clusterfuzz data

Review URL: https://codereview.chromium.org/1886423002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#387505}

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