Test no warn or crash in nullary override

Author: som-snytt

test/files/pos/t12851/C_2.scala

@@ -0,0 +1,2 @@
+// scalac: -Werror
+class C extends T2

test/files/pos/t12851/T_1.scala

@@ -0,0 +1,7 @@
+
+trait T1 {
+  def f: Int
+}
+trait T2 extends T1 {
+  def f() = 42
+}