cmd/gc: fix array index out of bounds error message

The error message was previously off by one in all cases.

Fixes #7150.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/65850043

src/cmd/gc/typecheck.c

@@ -2503,7 +2503,7 @@ typecheckcomplit(Node **np)
 				len = i;
 				if(t->bound >= 0 && len > t->bound) {
 					setlineno(l);
-					yyerror("array index %d out of bounds [0:%d]", len, t->bound);
+					yyerror("array index %d out of bounds [0:%d]", len-1, t->bound);
 					t->bound = -1;	// no more errors
 				}
 			}

test/fixedbugs/issue7150.go

+// errorcheck
+
+// 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.
+
+// issue 7150: array index out of bounds error off by one
+
+package main
+
+func main() {
+	_ = [0]int{-1: 50}              // ERROR "array index must be non-negative integer constant"
+	_ = [0]int{0: 0}                // ERROR "array index 0 out of bounds \[0:0\]"
+	_ = [0]int{5: 25}               // ERROR "array index 5 out of bounds \[0:0\]"
+	_ = [10]int{2: 10, 15: 30}      // ERROR "array index 15 out of bounds \[0:10\]"
+	_ = [10]int{5: 5, 1: 1, 12: 12} // ERROR "array index 12 out of bounds \[0:10\]"
+}