Correctly handle return code of Handle_Write()

There have been code paths that ignored the return code of Handle_Write()
when sending "notice auth" messages to new clients connecting to the
server. But because Handle_Write() would have closed the client connection
again if an error occurred, this would have resulted in new errors and
assert()'s later on that could have crashed the server (denial of service).

Only setups having the configuration option "NoticeAuth" enabled are
affected, which is not the default.

CVE-2013-5580.

(cherry picked from commit 309122017ebc6fff039a7cab1b82f632853d82d5)
ngIRCd Release 20.3