OutgoingEmail: Fix breakage introduced in add() method

In change Id79e13f5a the add method was refactored to check
for valid emails. During this refactoring one statement was
missed. It was not caught on this branch because there is no
acceptance test, and manual tests were only done for the
negative case (ivalid address, address prohibited).

The problem was caught when merging up to stable-2.12, where
there is an accepance test for mail messages.

Change-Id: Ie437f5d733c5a9d1015f0322c6e04055a73eb9b5

gerrit-server/src/main/java/com/google/gerrit/server/mail/OutgoingEmail.java

@@ -350,7 +350,7 @@ public abstract class OutgoingEmail {
         log.warn("Not emailing " + addr.email + " (invalid email address)");
       } else if (!args.emailSender.canEmail(addr.email)) {
         log.warn("Not emailing " + addr.email + " (prohibited by allowrcpt)");
-      } else {
+      } else if (smtpRcptTo.add(addr)) {
         switch (rt) {
           case TO:
             ((EmailHeader.AddressList) headers.get(HDR_TO)).add(addr);