Fix an invalid use of ::back() on an newly emptied vector. Also tighten
up several places where we never expect to have NULL pointers to assert
early.

This fixes a valgrind error within CorrectTypo, but not the
non-determinism.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@134032 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed