Send_Message: Fix handling of "empty" targets
Clients can specify multiple targets for the "PRIVMSG", "NOTICE", and
"SQUERY" commands, separated by commas (e. g. "PRIVMSG a,#b,c :text").
Since commit
49ab79d0 ("Limit the number of message targes, and suppress
duplicates"), ngIRCd crashed when the client sent the separator character
only as target(s), e. g. "," or ",,,," etc.!
This patch fixes the bug and adds a test case for this issue.
Thanks to Florian Westphal <fw@strlen.de> for spotting the issue!