must not hang if process doesn't exist, and we don't have an
existing wait message for it.

Reference: /n/atom/patch/applied2013/apewait4gone
Date: Tue Sep 17 16:05:42 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/ape/lib/ap/plan9/wait.c	Tue Sep 17 16:05:26 2013
+++ /sys/src/ape/lib/ap/plan9/wait.c	Tue Sep 17 16:05:26 2013
@@ -123,7 +123,7 @@
 		if(options & WNOHANG){
 			snprintf(pname, sizeof(pname), "/proc/%d/wait", getpid());
 			d = _dirstat(pname);
-			if(d != nil && d->length == 0){
+			if(d == nil || d->length == 0){
 				free(d);
 				return 0;
 			}