[analyzer] Fix a crash in RetainCountChecker - we should not rely on CallEnter::getCallExpr to return non-NULL

We get a CallEnter with a null expression, when processing a destructor. All other users of
CallEnter::getCallExpr work fine with null as return value.

(Addresses PR15832, Thanks to Jordan for reducing the test case!)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180234 91177308-0d34-0410-b5e6-96231b3b80d8