[analyzer] CStringChecker.cpp: Remove the duplicated check about null dereference on dest-buffer or src-buffer.
[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: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@326782 91177308-0d34-0410-b5e6-96231b3b80d8

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