Silence valgrind memcheck error by memset()-ing pkt to 0

valgrind reports:

    Syscall param write(buf) points to uninitialised byte(s)

In this case, it is a false positive, but adding this single memset at
attach start-up means the program is now clean when run with "valgrind
--trace-children=yes dtach -c ...", which can make future debugging
easier.

attach.c

@@ -190,6 +190,7 @@ attach_main(int noerror)
 	write(1, "\33[H\33[J", 6);
 
 	/* Tell the master that we want to attach. */
+	memset(&pkt, 0, sizeof(struct packet));
 	pkt.type = MSG_ATTACH;
 	write(s, &pkt, sizeof(struct packet));