[Bash-autocomplete] Fix crash when invoking --autocomplete without value.

Summary:
Currently clang segfaults when invoked with `clang --autocomplete=`.
This patch adds the necessary boundary checks and some tests for corner cases like this.

Reviewers: yamaguchi

Reviewed By: yamaguchi

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D37465

llvm-svn: 312533