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

gdb/ChangeLog

@@ -1,3 +1,10 @@
+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.
+
 2019-12-01  Tom Tromey  <tom@tromey.com>
 
 	* tui/tui.c (tui_enable): Call tui_update_variables earlier.

gdb/Makefile.in

@@ -442,6 +442,7 @@ SUBDIR_UNITTESTS_SRCS = \
 	unittests/string_view-selftests.c \
 	unittests/style-selftests.c \
 	unittests/tracepoint-selftests.c \
+	unittests/tui-selftests.c \
 	unittests/unpack-selftests.c \
 	unittests/utils-selftests.c \
 	unittests/vec-utils-selftests.c \

gdb/tui/tui-winsource.c

@@ -103,6 +103,8 @@ tui_copy_source_line (const char **ptr, int line_no, int first_col,
 	  lineptr += skip_bytes;
 	  continue;
 	}
+      if (c == '\0')
+	break;
 
       ++lineptr;
       ++column;

gdb/unittests/tui-selftests.c

@@ -0,0 +1,48 @@
+/* Self tests for the TUI
+
+   Copyright (C) 2019 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include "defs.h"
+#include "gdbsupport/selftest.h"
+#include "tui/tui-winsource.h"
+
+namespace selftests {
+namespace tui {
+
+static void
+run_tests ()
+{
+  const char *text = "hello";
+  std::string result = tui_copy_source_line (&text, 0, 0, 50, 0);
+  SELF_CHECK (result == "hello");
+  SELF_CHECK (*text == '\0');
+
+  text = "hello\n";
+  result = tui_copy_source_line (&text, 0, 0, 3, 0);
+  SELF_CHECK (result == "hel");
+  SELF_CHECK (*text == '\0');
+}
+
+} /* namespace tui*/
+} /* namespace selftests */
+
+void
+_initialize_tui_selftest ()
+{
+  selftests::register_test ("tui", selftests::tui::run_tests);
+}