do not use pointers as truth values

Reference: /n/atom/patch/applied/nedptrcmp
Date: Wed May 21 22:45:47 CES 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/upas/ned/nedmail.c	Wed May 21 22:45:47 2014
+++ /sys/src/cmd/upas/ned/nedmail.c	Wed May 21 22:45:47 2014
@@ -604,7 +604,7 @@
 			highest = last->fileno;
 	for(i = highest + 1;; i++){
 		snprint(buf, sizeof buf, "%s/%d", parent->path, i);
-		if(!(d = dirstat(buf)))
+		if((d = dirstat(buf)) == nil)
 			break;
 		if((d->qid.type & QTDIR) == 0){
 			free(d);
@@ -2361,7 +2361,7 @@
 	char *av[128], *rpath, *subject, *from, *to, *cc;
 	int i, ai;
 
-	if(!m->from || !m->to || !m->cc){
+	if(m->from == nil || m->to == nil || m->cc == nil){
 		eprint("!bad message\n");
 		return nil;
 	}
@@ -2694,7 +2694,7 @@
 	char *p;
 
 	p = strrchr(s, c);
-	if(p && p > s) {
+	if(p != nil && p > s) {
 		*p = 0;
 		return p - 1;
 	}