Fix crash in "run" on macOS when gdb is not signed

Fix crash in "run" on macOS when gdb is not signed

On macOS, when gdb is not code-signed, it will throw an exception from
darwin_attach_pid.  However, gdb also then crashes:

thread.c:93: internal-error: struct thread_info *inferior_thread(): Assertion `tp' failed.

I think the problem here is that darwin_attach_pid does not clean up
inferior_ptid and inf->pid on failure.  This leads to a situation
where gdb tries to find a thread, but cannot.

In other cases, gdb would mourn the inferior at this point; but here
this is not possible because the target has not been pushed.  Instead
this patch works by simply updating the inferior and inferior_ptid on
failure.

Tested by building an unsigned gdb on macOS and trying to run an
inferior.

gdb/ChangeLog
2018-07-03  Tom Tromey  <tom@tromey.com>

	PR cli/23340:
	* darwin-nat.c (darwin_attach_pid): Reset inferior and
	inferior_ptid on error.