JET-69 Lookup equals() in a non-null type of the receiver.

idea/testData/checker/regression/Jet69.jet

@@ -0,0 +1,8 @@
+fun parse(cmd: String): Command? { return null  }
+
+fun Any.equals(other : Any?) : Boolean = this === other
+
+fun main(args: Array<String>) {
+    val command = parse("")
+    if (command == null) 1 // error on this line, but must be OK
+}