use nil were necessary

Reference: /n/atom/patch/applied/imap4dnil
Date: Sun May 25 03:40:36 CES 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/upas/imap4d/nlisttst.c	Sun May 25 03:40:24 2014
+++ /sys/src/cmd/upas/imap4d/nlisttst.c	Sun May 25 03:40:25 2014
@@ -1,8 +1,7 @@
 #include "nlist.c"
 
 char	username[] = "quanstro";
-//char	mboxdir[] = "/mail/box/quanstro/";
-char	mboxdir[] = "/n/coraid/sys/src/cmd/upas/imap4d/mdir";
+char	mboxdir[] = "/mail/box/quanstro/";
 Biobuf	bout;
 Bin	*parsebin;
 
--- /sys/src/cmd/upas/imap4d/list.c	Sun May 25 03:40:28 2014
+++ /sys/src/cmd/upas/imap4d/list.c	Sun May 25 03:40:29 2014
@@ -395,7 +395,7 @@
 	if((mb = mblock()) == nil)
 		return 0;
 	fd = opensubscribed();
-	if(fd < 0 || !(in = readfile(fd))){
+	if(fd < 0 || (in = readfile(fd)) == nil){
 		close(fd);
 		mbunlock(mb);
 		return 0;