Fix Format function to not cut of last character

This fixes broke game save

The beginning of a saved game looked like this:
00000000: 6c6f 6361 6c20 6f6c 6443 7265 6174 6555  local oldCreateU
00000010: 6e69 7420 3d20 4372 6561 7465 556e 6974  nit = CreateUnit
00000020: 006c 6f63 616c 206f 6c64 5365 7452 6573  .local oldSetRes
00000030: 6f75 7263 6573 4865 6c64 203d 2053 6574  ourcesHeld = Set
00000040: 5265 736f 7572 6365 7348 656c 6400 6c6f  ResourcesHeld.lo

The newlines are replaced by null-characters so games could not be
loaded again

CMakeLists.txt

@@ -535,6 +535,7 @@ set(stratagus_tests_SRCS
 	tests/main.cpp
 	tests/stratagus/test_action_built.cpp
 	tests/stratagus/test_depend.cpp
+	tests/stratagus/test_format.cpp
 	tests/stratagus/test_luacallback.cpp
 	tests/stratagus/test_missile_fire.cpp
 	tests/stratagus/test_trigger.cpp

src/include/stratagus.h

@@ -118,7 +118,8 @@ std::string Format(const char *format, Ts... args)
 #endif
 	const auto len = snprintf(nullptr, 0, format, args...);
 	std::string res(len, '\0');
-	snprintf(res.data(), res.size(), format, args...);
+	// len+1: writing a '\0' over the existing '\0' should be fine
+	snprintf(res.data(), len + 1, format, args...);
 #ifndef _MSC_VER
 # pragma GCC diagnostic pop
 #endif

tests/stratagus/test_format.cpp

@@ -0,0 +1,48 @@
+//       _________ __                 __
+//      /   _____//  |_____________ _/  |______     ____  __ __  ______
+//      \_____  \\   __\_  __ \__  \\   __\__  \   / ___\|  |  \/  ___/
+//      /        \|  |  |  | \// __ \|  |  / __ \_/ /_/  >  |  /\___ |
+//     /_______  /|__|  |__|  (____  /__| (____  /\___  /|____//____  >
+//             \/                  \/          \//_____/            \/
+//  ______________________                           ______________________
+//                        T H E   W A R   B E G I N S
+//         Stratagus - A free fantasy real time strategy game engine
+//
+/**@name test_format.cpp - Test file for function Format. */
+//
+//      (c) Copyright 2024 by Matthias Schwarzott
+//
+//      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; only version 2 of the License.
+//
+//      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, write to the Free Software
+//      Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+//      02111-1307, USA.
+//
+
+#include <doctest.h>
+
+#include "stratagus.h"
+
+TEST_CASE("Format")
+{
+	CHECK("" == Format(""));
+	CHECK("1" == Format("1"));
+	CHECK("12" == Format("12"));
+	CHECK("12\n" == Format("12\n"));
+	CHECK("abc\n" == Format("%s", "abc\n"));
+	CHECK("abc\n" == Format("%s\n", "abc"));
+	CHECK("12345" == Format("%d", 12345));
+	CHECK("         x" == Format("%*s", 10, "x"));
+	std::string result = Format("%*s", 100, "x");
+	CHECK(result[98] == ' ');
+	CHECK(result[99] == 'x');
+	CHECK(result.size() == 100);
+}