Topic: Ensure do315's response is for a channel in our state object

Signed-off-by: James Vega <jamessan@users.sourceforge.net>
(cherry picked from commit 44eb449ba4)

Signed-off-by: Daniel Folkinshteyn <nanotube@users.sourceforge.net>

plugins/Topic/plugin.py

@@ -236,7 +236,9 @@ class Topic(callbacks.Plugin):
     def do315(self, irc, msg):
         # Try to restore the topic when not set yet.
         channel = msg.args[1]
-        c = irc.state.channels[channel]
+        c = irc.state.channels.get(channel)
+        if c is None:
+            return
         if irc.nick not in c.ops and 't' in c.modes:
             self.log.debug('Not trying to restore topic in %s. I\'m not opped '
                                'and %s is +t.', channel, channel)