Avoid crash in print_ada_task_info

Avoid crash in print_ada_task_info

In MI mode, print_ada_task_info can crash in find_thread_ptid when
trying to print an Ada task that is no longer alive.  This patch
avoids the problem by checking for this case.

Because this is Ada-specific, and because Joel approved it internally,
I am checking it in.

gdb/ChangeLog
2019-11-22  Tom Tromey  <tromey@adacore.com>

	* ada-tasks.c (ada_task_is_alive): Make parameter const.
	(print_ada_task_info): Don't try to fetch thread id if task is not
	alive.

gdb/gdbserver/ChangeLog
2019-11-22  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/tasks.exp: Add -ada-task-info regression test.
	* gdb.ada/tasks/foo.adb: Add another stopping location.

Change-Id: If25eae6507eebb7537eb8adbcbaa1fc1eec88f5c