CIndex: initialise TU

TU is not guaranteed to be initialised in all cases.  In particular if CIdx or
ast_filename is NULL (or if &TU is NULL), then clang_createTranslationUnit2 will
not initialise the out parameter out_TU.  This is followed by an assertion check
which may perform a branch based on unitialised memory.

Caught by scan-build.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201628 91177308-0d34-0410-b5e6-96231b3b80d8