Fix an dangerous substr() call
Fix an dangerous substr() call

A malicious IRCd could send a WHO reply for a nick which consisted completely
out of prefix characters (thus an empty nick). In this case
std::string::find_first_of() would return std::string::npos. This argument would
make std::string::substr() throw an exception and kill the process.

Signed-off-by: Uli Schlachter <346c0ad49ac8427b61b3b55abbe37dc0ebd524f2@znc.in>

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