Fix "thread apply all" with exited threads

Fix "thread apply all" with exited threads

I noticed that "thread apply all" sometimes crashes.

The problem is that thread_apply_all_command doesn take exited threads
into account, and we qsort and then walk more elements than there
really ever were put in the array.  Valgrind shows:

 The current thread <Thread ID 3> has terminated.  See `help thread'.
 (gdb) thread apply all p 1

 Thread 1 (Thread 0x7ffff7fc2740 (LWP 29579)):
 $1 = 1
 ==29576== Use of uninitialised value of size 8
 ==29576==    at 0x639CA8: set_thread_refcount (thread.c:1337)
 ==29576==    by 0x5C2C7B: do_my_cleanups (cleanups.c:155)
 ==29576==    by 0x5C2CE8: do_cleanups (cleanups.c:177)
 ==29576==    by 0x63A191: thread_apply_all_command (thread.c:1477)
 ==29576==    by 0x50374D: do_cfunc (cli-decode.c:105)
 ==29576==    by 0x506865: cmd_func (cli-decode.c:1893)
 ==29576==    by 0x7562CB: execute_command (top.c:476)
 ==29576==    by 0x647DA4: command_handler (event-top.c:494)
 ==29576==    by 0x648367: command_line_handler (event-top.c:692)
 ==29576==    by 0x7BF7C9: rl_callback_read_char (callback.c:220)
 ==29576==    by 0x64784C: rl_callback_read_char_wrapper (event-top.c:171)
 ==29576==    by 0x647CB5: stdin_event_handler (event-top.c:432)
 ==29576==
 ...

This can happen easily today as linux-nat.c/linux-thread-db.c are
forgetting to purge non-current exited threads.  But even with that
fixed, we can always do "thread apply all" with an exited thread
selected, which won't be deleted until the user switches to another
thread.  That's what the test added by this commit exercises.

Tested on x86_64 Fedora 20.

gdb/ChangeLog:
2015-03-24  Pedro Alves  <palves@redhat.com>

	* thread.c (thread_apply_all_command): Take exited threads into
	account.

gdb/testsuite/ChangeLog:
2015-03-24  Pedro Alves  <palves@redhat.com>

	* gdb.threads/no-unwaited-for-left.exp: Test "thread apply all".