Address undefined behaviour in StringPiece code.
Address undefined behaviour in StringPiece code.

http://en.cppreference.com/w/c/string/byte/memcmp says: "The behavior
is undefined if access occurs beyond the end of either object pointed
to by lhs and rhs. The behavior is undefined if either lhs or rhs is a
null pointer."

Fixes #129.

Change-Id: Ibc73788dc7414c08fc1427d8dc06ad542de9798c
Reviewed-on: https://code-review.googlesource.com/10292
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2