[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
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@326782
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96231b3b80d8