gdbserver/Linux: unbreak thread event randomization

Wanting to make sure the new continue-pending-status.exp test tests
both cases of threads 2 and 3 reporting an event, I added counters to
the test, to make it FAIL if events for both threads aren't seen.
Assuming a well behaved backend, and given a reasonable number of
iterations, it should PASS.

However, running that against GNU/Linux gdbserver, I found that
surprisingly, that FAILed.  GDBserver always reported the breakpoint
hit for the same thread.

Turns out that I broke gdbserver's thread event randomization
recently, with git commit 582511be ([gdbserver] linux-low.c: better
starvation avoidance, handle non-stop mode too).  In that commit I
missed that the thread structure also has a status_pending_p field...
The end result was that count_events_callback always returns 0, and
then if no thread is stepping, select_event_lwp always returns the
event thread.  IOW, no randomization is happening at all.  Quite
curious how all the other changes in that patch were sufficient to fix
non-stop-fair-events.exp anyway even with that broken.

Tested on x86_64 Fedora 20, native and gdbserver.

gdb/gdbserver/ChangeLog:
2015-03-19 Pedro Alves  <palves@redhat.com>

	* linux-low.c (count_events_callback, select_event_lwp_callback):
	Use the lwp's status_pending_p field, not the thread's.

gdb/testsuite/ChangeLog:
2015-03-19  Pedro Alves  <palves@redhat.com>

	* gdb.threads/continue-pending-status.exp (saw_thread_2)
	(saw_thread_3): New globals.
	(top level): Increment them when an event for the corresponding
	thread is seen.
	(no thread starvation): New test.

gdb/linux-nat.c

@@ -2841,6 +2841,7 @@ select_event_lwp (ptid_t filter, struct lwp_info **orig_lp, int *status)
 
       /* First see how many events we have.  */
       iterate_over_lwps (filter, count_events_callback, &num_events);
+      gdb_assert (num_events > 0);
 
       /* Now randomly pick a LWP out of those that have had
 	 events.  */