don't crash issuing spurious nilary-overrides-nullary warning

fixes scala/bug#12851

Author: SethTisue

src/compiler/scala/tools/nsc/typechecker/RefChecks.scala

@@ -461,7 +461,8 @@ abstract class RefChecks extends Transform {
               }
               else if (other.paramss.isEmpty && !member.paramss.isEmpty &&
                 !javaDetermined(member) && !member.overrides.exists(javaDetermined) &&
-                !member.hasAnnotation(BeanPropertyAttr) && !member.hasAnnotation(BooleanBeanPropertyAttr)
+                !member.hasAnnotation(BeanPropertyAttr) && !member.hasAnnotation(BooleanBeanPropertyAttr) &&
+                member.pos.isDefined  // scala/bug#12851
               ) {
                 val msg = "method with a single empty parameter list overrides method without any parameter list"
                 val namePos = member.pos.focus.withEnd(member.pos.point + member.decodedName.length)