Fix crash when TUI window creation fails

Fix crash when TUI window creation fails

If a TUI window is written in Python, and if the window construction
function fails, then gdb will crash.  This patch fixes the crash.

gdb/ChangeLog
2020-06-16  Tom Tromey  <tom@tromey.com>

	* python/py-tui.c (tui_py_window::~tui_py_window): Handle case
	where m_window==nullptr.

gdb/testsuite/ChangeLog
2020-06-16  Tom Tromey  <tom@tromey.com>

	* gdb.python/tui-window.py (failwin): New function.  Register it
	as a TUI window type.
	* gdb.python/tui-window.exp: Create new "fail" layout.  Test it.