test: add test that gccgo failed to compile

Gccgo failed to create the type descriptor for the type used to
allocate the nil value passed to append as the second argument when
append is called with only one argument.  Calling append with only one
argument is unusual but obviously should not cause a compiler crash.

Change-Id: I530821847dfd68f0302de6ca6a84dfbc79653935
Reviewed-on: https://go-review.googlesource.com/1692Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/bug493.go

+// compile
+
+// Copyright 2014 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.
+
+// Test case that gccgo failed to compile.
+
+package p
+
+func F() []string {
+	return []string{""}
+}
+
+var V = append(F())