Make TUI borders respect "set style enabled"

Make TUI borders respect "set style enabled"

When adding support for styling the TUI borders, I neglected to have
this code check cli_styling.  As a result, "set style enabled off"
does not affect the borders.

This patch fixes this oversight.  While doing this, I found that
running gdb without an executable, enabling the TUI, and then trying
"set style enabled off" would fail with the mysterious "No registers".
The fix for this is to use deprecated_safe_get_selected_frame in
tui_source_window_base::refill.

gdb/ChangeLog
2020-01-11  Tom Tromey  <tom@tromey.com>

	* tui/tui-wingeneral.c (box_win): Check cli_styling.
	* tui/tui-winsource.c (tui_source_window_base::refill): Use
	deprecated_safe_get_selected_frame.

Change-Id: I36acda25dd9014d994d366b4a0e8faee9d95d0f8