Avoid calling std::memcmp with nullptr

Summary:
UBSAN complains that this is undefined behavior.

We can assume that empty substring (N==1) always satisfy conditions. So
std::memcmp will be called only only for N > 1 and Str.size() > 0.

Reviewers: ruiu, zturner

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D26646

llvm-svn: 286910