cmd/gc: fix segfault in isgoconst.

Variables declared with 'var' have no sym->def.

Fixes #7794.

LGTM=rsc
R=golang-codereviews, bradfitz, rsc
CC=golang-codereviews
https://golang.org/cl/88360043

src/cmd/gc/const.c

@@ -1594,7 +1594,7 @@ isgoconst(Node *n)
 
 	case ONAME:
 		l = n->sym->def;
-		if(l->op == OLITERAL && n->val.ctype != CTNIL)
+		if(l && l->op == OLITERAL && n->val.ctype != CTNIL)
 			return 1;
 		break;
 	

test/fixedbugs/issue7794.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.
+
+package main
+
+func main() {
+	var a [10]int
+	const ca = len(a)
+}