Change tui_source_element::line to be a unique_xmalloc_ptr

Change tui_source_element::line to be a unique_xmalloc_ptr

This changes tui_source_element::line to be a unique_xmalloc_ptr,
removing some manual memory management.

gdb/ChangeLog
2019-09-20  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.h (~tui_source_element): Remove.
	(tui_source_element): Update.
	(struct tui_source_element) <line>: Now a unique_xmalloc_ptr.
	* tui/tui-winsource.c (tui_show_source_line): Update.
	* tui/tui-source.c (tui_source_window::set_contents): Update.
	* tui/tui-disasm.c (tui_disasm_window::set_contents): Update.