Fix undefined behavior when activating scripts

It turns out that the line `tstring=tstring[tstring.size()-1];` also appears once in Scripts.cpp.
This causes the game to segfault after activating a terminal with an empty line at the end of it.
I added a quick `if` around this line, and set `tstring` to an empty string when needed.

desktop_version/src/Scripts.cpp

@@ -40,7 +40,11 @@ void scriptclass::load(std::string t)
         }else if(scriptend==-1){
           //Find the end
           tstring=script.customscript[i];
-          tstring=tstring[tstring.size()-1];
+          if (tstring.size() > 0) {
+            tstring=tstring[tstring.size()-1];
+          } else {
+            tstring="";
+          }
           if(tstring==":"){
             scriptend=i;
           }