talloc_stack: make sure we never let talloc_tos() return ts->talloc_stack[-1]

talloc_stack: make sure we never let talloc_tos() return ts->talloc_stack[-1]

In smbd there's a small gab between TALLOC_FREE(frame); before
we call smbd_parent_loop() where we don't have a valid talloc stackframe.

smbd_parent_loop() calls talloc_stackframe() only within the while(1) loop.
As DEBUG(2,("waiting for connections")) uses talloc_tos() to construct
the time header for the debug message we crash on some systems.

metze
(cherry picked from commit 10ed809a1a31be50ce09142eb99b3a243ae8b940)