Fix redisplay of the current line in GDB TUI mode

Fix redisplay of the current line in GDB TUI mode

Without this change, when the current line is longer than the source
window width, redisplaying that line overwrites the window frame and
also portions of the next line.

gdb/ChangeLog:
2019-03-17  Eli Zaretskii  <eliz@gnu.org>

	* tui/tui-winsource.c (tui_set_is_exec_point_at): Call
	tui_refill_source_window instead of tui_refresh_win, to update the
	current execution line.  This fixes redisplay of the current line
	when stepping through very long lines with "next" or "step".