Make "file" clear TUI source window

I noticed that a plain "file" will leave the current source file in
the TUI source window.  Instead, I think, it should clear the source
window.  This patch implements this.

gdb/ChangeLog
2020-01-19  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.c (tui_update_source_windows_with_line):
	Handle case where symtab is null.

gdb/testsuite/ChangeLog
2020-01-19  Tom Tromey  <tom@tromey.com>

	* gdb.tui/main.exp: Add check for plain "file".

Change-Id: I8424acf837f1a47f75bc6a833d1e917d4c10b51e