[analyzer] CStringChecker.cpp: Remove the duplicated check about null...

[analyzer] CStringChecker.cpp: Remove the duplicated check about null dereference on dest-buffer or src-buffer.

Summary: `CheckBufferAccess()` calls `CheckNonNull()`, so there are some calls to `CheckNonNull()` that are useless.

Reviewers: dcoughlin, NoQ, xazax.hun, cfe-commits, george.karpenkov

Reviewed By: NoQ

Subscribers: szepet, rnkovacs, MTC, a.sidorin

Differential Revision: https://reviews.llvm.org/D44075

llvm-svn: 326782