[asan] Fix strncmp and strncasecmp interceptors

Summary:
In non-strict mode we will check memory access for both strings from beginning
to either:
  1. 0-char
  2. size
  3. different chars

In strict mode we will check from beginning to either:
  1. 0-char
  2. size

Previously in strict mode we always checked up to the 0-char.

Reviewers: kcc, eugenis

Subscribers: llvm-commits, kubabrecka

Differential Revision: https://reviews.llvm.org/D26574

llvm-svn: 286708