Don't read CompilerInstance fields that don't exist in ASTUnit

When transferring data from a CompilerInstance in an error path we need
to consider cases where the various fields are uninitialized.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206644 91177308-0d34-0410-b5e6-96231b3b80d8