Use start_line_or_addr in TUI windows

A few spots in the TUI source and disassembly windows referred to
content[0], where start_line_or_addr is equivalent.  This patch makes
this substitution.

gdb/ChangeLog
2019-12-20  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.c (tui_source_window_base::refill): Use
	start_line_or_addr.
	* tui/tui-source.c (tui_source_window::do_scroll_vertical): Use
	start_line_or_addr.
	* tui/tui-disasm.c (tui_disasm_window::do_scroll_vertical): Use
	start_line_or_addr.

Change-Id: I1fa807321cd7ad88b3cc5e41cc50f4d4e2d46271