Some more defensive programming

src/main/scala/millfork/parser/MfParser.scala

@@ -33,7 +33,8 @@ abstract class MfParser[T](fileId: String, input: String, currentDirectory: Stri
   def toAst: Parsed[Program] = {
     val parse = program.parse(input + "\n\n\n")
     parse match {
-      case _:Failure[_, _] =>
+      case _: Failure[_, _] =>
+        if (lastPosition.cursor >= 0 && lastPosition.cursor < input.length) {
           val c = input(lastPosition.cursor)
           if (c >= 0x100 || c < 0x20 || c == '`') {
             log.error("Invalid character U+%04X %s".format(c.toInt, Character.getName(c)), Some(lastPosition))
@@ -43,6 +44,7 @@ abstract class MfParser[T](fileId: String, input: String, currentDirectory: Stri
               case _ =>
             }
           }
+        }
       case _ =>
     }
     parse