removed all uses of undefined from code base

It's a code smell, can lead to hard to diagnose error messages.

Assistant/Threads/Committer.hs

@@ -196,7 +196,7 @@ maxCommitSize :: Int
 maxCommitSize = 5000
 
 {- Decide if now is a good time to make a commit.
- - Note that the list of changes has an undefined order.
+ - Note that the list of changes has a random order.
  -
  - Current strategy: If there have been 10 changes within the past second,
  - a batch activity is taking place, so wait for later.

Assistant/Threads/PairListener.hs

@@ -31,7 +31,7 @@ pairListenerThread urlrenderer = namedThread "PairListener" $ do
   where
 	{- Note this can crash if there's no network interface,
 	 - or only one like lo that doesn't support multicast. -}
-	getsock = multicastReceiver (multicastAddress $ IPv4Addr undefined) pairingPort
+	getsock = multicastReceiver (multicastAddress IPv4AddrClass) pairingPort
 		
 	go reqs cache sock = liftIO (getmsg sock []) >>= \msg -> case readish msg of
 		Nothing -> go reqs cache sock

Assistant/Threads/XMPPPusher.hs

@@ -78,4 +78,5 @@ selectNextPush lastpushedto l = go [] l
 		(Pushing clientid _)
 			| Just clientid /= lastpushedto -> (m, rejected ++ ms)
 		_ -> go (m:rejected) ms
-	go [] [] = undefined
+	go [] [] = error "empty push queue"
+