test/vp[89]_boolcoder_test: quiet msan warnings

the bitreaders may fill beyond what was written to the buffer as an
optimization. the data isn't used meaningfully, but it may trigger a
msan warning.

BUG=b/140939146

Change-Id: Id03cd203b8ee7ecaf6fdfe3f3c9f2ccfec527129
2 files changed