Fix latent bug in tui_copy_source_line

Fix latent bug in tui_copy_source_line

tui_copy_source_line has a bug, where it can advance past the
terminating \0 in its input string.  This patch fixes the bug and adds
a test case for this function.

gdb/ChangeLog
2019-12-01  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.c (tui_copy_source_line): Don't advance past
	\0.
	* unittests/tui-selftests.c: New file.
	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add tui-selftests.c.

Change-Id: I46cdabe6e57549983149b8f640cda5edd16fa260