[Lexer] Fix EOF action

Author: LuisHsu

src/lib/Util.cpp

@@ -73,7 +73,7 @@ std::string handle_indent(int indent, std::string content){
     if(indent >= 0){
         // measure indent
         size_t space_pos = content.find_first_not_of(" \t\n\r\v");
-        if(space_pos != std::string::npos){
+        if((space_pos != std::string::npos) && (space_pos != 0)){
             std::string spaces = content.substr(0, space_pos);
             content = content.substr(space_pos);
             content = std::regex_replace(content, std::regex(spaces), "\n" + std::string(indent, ' '));