Fix normal_stop latent bug

Fix normal_stop latent bug

TARGET_WAITKIND_NO_RESUMED doesn't have an associated event thread, so
we shouldn't be referring to inferior_thread() assuming it points to
one.

This was caught on the multi-target branch, where we always switch to
no-thread-selected whenever we start handling an event, exactly to
catch places that incorrectly use "inferior_ptid/inferior_thread()"
without switching to the right event thread / target.

Here, on the branch, we assert in inferior_thread() because
TARGET_WAITKIND_NO_RESUMED doesn't have an associated event thread, so
inferior_ptid is still null_ptid.

gdb/ChangeLog:
2019-03-07  Pedro Alves  <palves@redhat.com>

	* infrun.c (normal_stop): Also check for
	TARGET_WAITKIND_NO_RESUMED before referring to inferior_thread().