[dev] MessageCache notices with HEAD

[Chuck Hagenbuch]

With latest CVS, I'm getting one of these:

/var/www/horde/imp/lib/IMAP/MessageCache.php:486: Undefined index: msgcache

For every message that's displayed. Looks like this will happen when  
the msgcache is turned off (mine got turned off because of the config  
changes - it's fine with it turned back on), because the MessageCache  
constructor aborts if caching is turned off, but it's still possible  
for other MessageCache functions to be called.

-chuck