Avoid find_thread_ptid with null_ptid

With a following patch, find_thread_ptid will first find the inferior
for the passed-in ptid, using find_inferior_pid, and then look for the
thread in that inferior's thread list.  If we pass down null_ptid to
find_thread_ptid then that means we'll end up passing 0 to
find_inferior_pid, which hits this assertion:

>   struct inferior *
>   find_inferior_pid (int pid)
>   {
>     struct inferior *inf;
>
>     /* Looking for inferior pid == 0 is always wrong, and indicative of
>	a bug somewhere else.  There may be more than one with pid == 0,
>	for instance.  */
>     gdb_assert (pid != 0);

This patch prepares for the change, by avoiding passing down null_ptid
to find_thread_ptid or to functions that naturally use it, such as the
target_pid_to_str call in inferior.c:add_inferior.  In that latter
case, the patch changes GDB output,

from:
 (gdb) add-inferior
 [New inferior 2 (process 0)]

to:
 (gdb) add-inferior
 [New inferior 2]

which seems like a good change to me.  It might not even make sense to
talk about "process" for the current target, for example.

The python_on_normal_stop change ends up avoiding looking up the
same thread twice (inferior_thread also does a look up).

gdb/ChangeLog:
2018-11-22  Pedro Alves  <palves@redhat.com>

	* cli/cli-interp.c (cli_on_user_selected_context_changed): Use
	inferior_thread instead of find_thread_ptid, and only when
	inferior_ptid is not null_ptid.
	* inferior.c (add_inferior): Don't include target_pid_to_str
	output when the inferior is not started.
	* python/py-inferior.c (python_on_normal_stop): Don't use
	find_thread_ptid.
	(tui_on_user_selected_context_changed): Use inferior_thread
	instead of find_thread_ptid, and only when inferior_ptid is not
	null_ptid.

gdb/python/py-inferior.c

@@ -86,8 +86,8 @@ python_on_normal_stop (struct bpstats *bs, int print_frame)
   if (!gdb_python_initialized)
     return;
 
-  if (!find_thread_ptid (inferior_ptid))
-      return;
+  if (inferior_ptid == null_ptid)
+    return;
 
   stop_signal = inferior_thread ()->suspend.stop_signal;