Simplify uses of thread_to_thread_object

Simplify uses of thread_to_thread_object

An review by Simon of an earlier showed a few spots related to
thread_to_thread_object that could be simplified.  This also detected
a latent bug, where thread_to_thread_object was inconsistent about
setting the Python exception before a NULL return.

Tested on x86-64 Fedora 28.

gdb/ChangeLog
2018-09-16  Tom Tromey  <tom@tromey.com>

	* python/py-threadevent.c (py_get_event_thread): Simplify.
	* python/py-inferior.c (infpy_thread_from_thread_handle):
	Return immediately after calling thread_to_thread_object.  Use
	Py_RETURN_NONE.
	(thread_to_thread_object): Set the exception on a NULL return.