fix peek_si to enable 1 byte show existing frames.

The test for this is in test vector code ( show existing frames will
fail ).  I can't check it in disabled as I'm changing the generic
test code to do this:

https://gerrit.chromium.org/gerrit/#/c/70569/


Change-Id: I5ab324f0cb7df06316a949af0f7fc089f4a3d466