Add bug143 to demonstrate bug with "v, ok :=" on pointers to maps.

R=r
APPROVED=r
DELTA=26  (26 added, 0 deleted, 0 changed)
OCL=27651
CL=27651

test/bugs/bug143.go

+// $G $D/$F.go || echo BUG should compile
+
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+type myMap map[string] int;
+
+func main() {
+	m := make(myMap);
+	mp := &m;
+
+	{
+		x, ok := m["key"];
+	}
+	{
+		x, ok := (*mp)["key"]
+	}
+	{
+		x, ok := mp["key"]
+	}
+}
+
+/*
+ * bug143.go:19: assignment count mismatch: 2 = 1
+ * bug143.go:18: x: undefined
+ * bug143.go:18: ok: undefined
+ */