expand: don't panic on bad array indexing

If an index goes past the end, an empty result is returned.

If an index is negative, we should error.

Both of these cases used to panic.

expand/param.go

@@ -313,7 +313,10 @@ func (cfg *Config) varInd(vr Variable, idx syntax.ArithmExpr) (string, error) {
 		if err != nil {
 			return "", err
 		}
-		if len(vr.List) > 0 {
+		if i < 0 {
+			return "", fmt.Errorf("negative array index")
+		}
+		if i < len(vr.List) {
 			return vr.List[i], nil
 		}
 	case Associative:

interp/interp_test.go

@@ -2003,6 +2003,14 @@ set +o pipefail
 		`a=b; echo "${a[@]}"`,
 		"b\n",
 	},
+	{
+		`a=(b); echo ${a[3]}`,
+		"\n",
+	},
+	{
+		`a=(b); echo ${a[-2]}`,
+		"negative array index\nexit status 1 #JUSTERR",
+	},
 
 	// associative arrays
 	{