BUG/MINOR: dns: dns_connect_server must return -1 unsupported nameserver's type

This patch fix returns code in case of dns_connect_server is called
on unsupported type (which should not happen). Doing this we have
the warranty that after a return 0 the fd is never -1.

This patch should fix github issues #1127, #1128 and #1130

src/dns.c

@@ -75,8 +75,10 @@ static int dns_connect_nameserver(struct dns_nameserver *ns)
 		dgram->t.sock.fd = fd;
 		fd_insert(fd, dgram, dgram_fd_handler, MAX_THREADS_MASK);
 		fd_want_recv(fd);
+		return 0;
 	}
-	return 0;
+
+	return -1;
 }
 
 /* Sends a message to a name server