Avoid crash in ravenscar_thread_target::wait

An earlier patch caused a Ravenscar regression in
ravenscar_thread_target::wait.  In particular, add_active_thread can
return NULL when the runtime is not initialized.

gdb/ChangeLog
2020-08-07  Tom Tromey  <tromey@adacore.com>

	* ravenscar-thread.c (ravenscar_thread_target::wait): Check
	runtime_initialized.