[analyzer] Prevent AnalyzerStatsChecker from crash

The checker marks the locations where the analyzer creates sinks. However, it
can happen that the sink was created because of a loop which does not contain
condition statement, only breaks in the body. The exhausted block is the block
which should contain the condition but empty, in this case.
This change only emits this marking in order to avoid the undefined behavior.

Differential Revision: https://reviews.llvm.org/D42266



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@325693 91177308-0d34-0410-b5e6-96231b3b80d8