vp9: Fix potential SEGV in decoder_peek_si_internal

decoder_peek_si_internal could potentially read more bytes than
what actually exists in the input buffer. We check for the buffer
size to be at least 8, but we try to read up to 10 bytes in the
worst case. A well crafted file could thus cause a segfault.
Likely change that introduced this bug was:
https://chromium-review.googlesource.com/#/c/70439 (git hash:
7c43fb67)

BUG=chromium:621095

Change-Id: Id74880cfdded44caaa45bbdbaac859c09d3db752