cmd/compile: prevent panic while formatting func(...T) with unknown T

Compile:

package p

var f = func(...A)


Before this CL:

x.go:3:13: type %!v(PANIC=runtime error: invalid memory address or nil pointer dereference) is not an expression
x.go:3:17: undefined: A


After this CL:

x.go:3:13: type func(...<T>) is not an expression
x.go:3:17: undefined: A


Found with go-fuzz.

Fixes #20233

Change-Id: Ibb232b3954c4091071440eba48b44c4022a8083f
Reviewed-on: https://go-review.googlesource.com/42610
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/fmt.go

@@ -1754,7 +1754,11 @@ func fldconv(f *types.Field, flag FmtFlag, mode fmtMode, depth int) string {
 
 	var typ string
 	if f.Isddd() {
-		typ = "..." + tmodeString(f.Type.Elem(), mode, depth)
+		var et *types.Type
+		if f.Type != nil {
+			et = f.Type.Elem()
+		}
+		typ = "..." + tmodeString(et, mode, depth)
 	} else {
 		typ = tmodeString(f.Type, mode, depth)
 	}

test/fixedbugs/issue20233.go

+// errorcheck
+
+// Copyright 2017 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.
+
+// Issue 20233: panic while formatting an error message
+
+package p
+
+var f = func(...A) // ERROR "type func(....*) is not an expression" ERROR "undefined: A"