manager: Make challenge work with allowmultiplelogin=no

Prior to this patch, challenge would yield a multiple logins error if used
without providing the username (which isn't really supposed to be an argument
to challenge) if allowmultiplelogin was set to no because allowmultiplelogin
finds a user with a zero length login name. This check is simply disabled for
the challenge action when the username is empty by this patch.

(closes issue ASTERISK-20677)
Reported by: Vladimir
Patches:
    challenge_action_nomultiplelogin.diff uploaded by Jonathan Rose (license 6182)


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@376725 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/manager.c

@@ -4696,7 +4696,7 @@ static int process_message(struct mansession *s, const struct message *m)
 			|| !strcasecmp(action, "Challenge"))) {
 		user = astman_get_header(m, "Username");
 
-		if (check_manager_session_inuse(user)) {
+		if (!ast_strlen_zero(user) && check_manager_session_inuse(user)) {
 			report_session_limit(s);
 			sleep(1);
 			mansession_lock(s);