Fix inferior_thread attribute in new_thread event

Commit 72ee03ff58 fixed a use-after-move bug in add_thread_object, but
it changed the inferior_thread attribute to contain the inferior instead
of the actual thread.
This now uses the thread_obj in its new location instead.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28429
2 files changed