cmd/gc: fix overflow in array bounds calculation.

Fixes #5609.

R=golang-dev, remyoudompheng, r
CC=golang-dev
https://golang.org/cl/9757045

src/cmd/gc/typecheck.c

@@ -420,12 +420,12 @@ reswitch:
 				goto error;
 			}
 			t->bound = mpgetfix(v.u.xval);
-			if(t->bound < 0) {
-				yyerror("array bound must be non-negative");
-				goto error;
-			} else if(doesoverflow(v, types[TINT])) {
+			if(doesoverflow(v, types[TINT])) {
 				yyerror("array bound is too large"); 
 				goto error;
+			} else if(t->bound < 0) {
+				yyerror("array bound must be non-negative");
+				goto error;
 			}
 		}
 		typecheck(&r, Etype);

test/fixedbugs/issue5609.go

+// errorcheck
+
+// Copyright 2013 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 5609: overflow when calculating array size
+
+package pkg
+
+const Large uint64 = 18446744073709551615
+
+var foo [Large]uint64 // ERROR "array bound is too large"