Add unsigned char static_cast to argument of std::isdigit()

Apparently it results in Undefined Behavior if the argument given isn't
representable as an unsigned char. This means that (potentially) plain
char and signed chars could be unsafe to use as well.

It's rare that this could happen in practice, though. std::isdigit() is
only used by is_positive_num() which is only used by find_tag(), so
someone would have to deliberately put something crazy after an `&#` in
a custom level file in order for this to happen. Still, better to be
safe than sorry and all.

desktop_version/src/UtilityClass.cpp

@@ -213,7 +213,7 @@ bool is_positive_num(const std::string& str)
 {
 	for (size_t i = 0; i < str.length(); i++)
 	{
-		if (!std::isdigit(str[i]))
+		if (!std::isdigit(static_cast<unsigned char>(str[i])))
 		{
 			return false;
 		}