Fix out-of-index issue in rewrite plugin (#5462)

This PR fixes another out-of-index issue in rewrite to avoid
security vuln.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

plugin/rewrite/rewrite.go

@@ -100,6 +100,9 @@ func newRule(args ...string) (Rule, error) {
 	switch arg0 {
 	case Continue:
 		mode = Continue
+		if len(args) < 2 {
+			return nil, fmt.Errorf("continue rule must begin with a rule type")
+		}
 		ruleType = strings.ToLower(args[1])
 		expectNumArgs = len(args) - 1
 		startArg = 2