Fix some signed integer UB in RNG and related code

src/common/tpt-rand.cpp

@@ -35,7 +35,7 @@ unsigned int RNG::operator()()
 int RNG::between(int lower, int upper)
 {
 	unsigned int r = next();
-	return static_cast<int>(r % (upper - lower + 1)) + lower;
+	return static_cast<int>(r % ((unsigned int)(upper) - (unsigned int)(lower) + 1U)) + lower;
 }
 
 bool RNG::chance(int nominator, unsigned int denominator)

src/lua/LuaScriptInterface.cpp

@@ -200,7 +200,7 @@ static int mathRandom(lua_State *l)
 	{
 		luaL_error(l, "interval is empty");
 	}
-	if (upper - lower + 1)
+	if ((unsigned int)(upper) - (unsigned int)(lower) + 1U)
 	{
 		lua_pushinteger(l, rng.between(lower, upper));
 	}