Fix a null access due to assigning nil to the member memberships map

Even though the memberships map has type map[string]*memberships
(with memberships being defined as []membership), the default value for
that map should not be `nil` but a pointer to a nil slice.

This fixes a segfault on some servers before user channel prefixes are
sent.

upstream.go

@@ -692,7 +692,7 @@ func (uc *upstreamConn) handleMessage(msg *irc.Message) error {
 				if err != nil {
 					return err
 				}
-				ch.Members[msg.Prefix.Name] = nil
+				ch.Members[msg.Prefix.Name] = &memberships{}
 			}
 
 			chMsg := msg.Copy()