Fix secret channel information leak

/who on a secret channel that the user is not a member of
now returns proper RPL_ENDOFWHO_MSG instead of nothing.

src/testsuite/ngircd-test.conf

@@ -28,5 +28,10 @@
 	Name = #TopicChannel
 	Modes = t
 	Topic = the topic
+	
+[CHANNEL]
+	Name = #SecretChannel
+	Modes = s
+	Topic = A secret Channel
 
 # -eof-

src/testsuite/who-test.e

@@ -152,6 +152,16 @@ expect {
 	timeout { exit 1 }
 	":ngircd.test.server 352 nick \* * localhost ngircd.test.server nick H\* :0 Real Name"
 }
+expect {
+	timeout { exit 1 }
+	"315"
+}
+
+send "who #SecretChannel\r"
+expect {
+	timeout { exit 1 }
+	"315"
+}
 
 send "quit\r"
 expect {