[RFA] Remove a cleanup from parse_expression_for_completion

[Tom Tromey]

>>>>> "Tom" == Tom Tromey <tom@tromey.com> writes:

Tom> Here's an updated patch.  Let me know what you think.

Actually, this patch is wrong, and I didn't realize it until this
revision.  There was a failure in the buildbot report that I didn't
notice in the noise :(

There is a case in here where the NULL/empty string distinction matters.
So I think we can't use std::string but must instead use
unique_xmalloc_ptr.  I'll update the patch.

Tom