Fix a few more crashes.
One has to do with the string decoder.  In that case, just return an empty string and print the offending characters to the log.  The other has to do with a backlog request event being null.  In that case, just ignore the request.  The last was discovered by running IRCfuzz.c and was fixed by not throwing exceptions from CoreConnService.
Merge pull request #138 from mamarley/morecrashfixes

Fix a few more crashes.
Merge pull request #137 from mamarley/netsplitjoinracebugfix

Fix the join race in the netsplit join handling code too.
Merge pull request #135 from mamarley/joinracebugfix

Send requests for user and channel init data out of CoreConnService
Merge pull request #136 from mamarley/bufferhighlightcolorfix

Use the same buffer highlight color that the regular Quassel client uses