gdb/tui: Use cleanups to free string copies.

gdb/tui: Use cleanups to free string copies.

In parse_scrolling_args it is possible for a string copy to leak if an
error occurs.  Switching to using a cleanup fixes this leak.

In tui_set_win_height the string can't be leaked, but switching to using
a cleanup guards against the possibility that a leak could be introduced
in the future (by adding an error somewhere in the call stack).

gdb/ChangeLog:

	* tui/tui-win.c (tui_set_win_height): Use a cleanup to free the
	string copy.
	(parse_scrolling_args): Likewise.