[analyzer] CString Checker: Do not split the path unless the user

specifically checks for equality to null.

Enforcing this general practice, which keeps the analyzer less
noisy, in the CString Checker. This change suppresses "Assigned value is
garbage or undefined" warning in the added test case.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@156085 91177308-0d34-0410-b5e6-96231b3b80d8