Fix use-after-move bug in add_thread_object

commit 05b08ac1 ("Reduce manual reference counting in
py-inferior.c") introduced a use-after-move bug in add_thread_object,
causing a test suite failure.  This patch fixes the bug.

Tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-07-11  Tom Tromey  <tromey@adacore.com>

	* python/py-inferior.c (add_thread_object): Don't use thread_obj
	after it has been moved.