fix varint decoding in case the first buffer is smaller than encoded varint

fix varint decoding in case the first buffer is smaller than encoded varint

Summary: This is not a super clean solution, given that I have just copied the code over form Varint.h. Another way would be to add a peek(IOBuf&, size_t) method to CursorBase and use that to peek deeper into the chain.

Test Plan: unit tests

Reviewed By: tulloch@fb.com

Subscribers: trunkagent, njormrod, dcapel, benr, folly-diffs@

FB internal diff: D1689872

Signature: t1:1689872:1416964989:b7f12a2686233f161401288ffcb8c51926b01fdf