use more consistent types

clib/ml_string.cpp

@@ -93,7 +93,7 @@ std::string string_substring(const std::string & str, long pos, long count) {
 	return str.substr(start_pos, count);
 }
 
-size_t string_find_substr(const std::string & str, const std::string & pattern, long pos) {
+size_t string_find_substr(const std::string & str, const std::string & pattern, size_t pos) {
 	if (pos >= str.size()) {
 		throw std::invalid_argument("Invalid parameter(s) for string-find.");
 	}