return -1 on queue closed; don't queue crud.

Reference: /n/atom/patch/applied2013/qiwriteclosed
Date: Tue Oct 22 10:20:10 CES 2013
Signed-off-by: quanstro@quanstr.net

--- /sys/src/nix/port/qio.c	Tue Oct 22 10:19:51 2013
+++ /sys/src/nix/port/qio.c	Tue Oct 22 10:19:53 2013
@@ -1377,6 +1377,11 @@
 
 		ilock(q);
 
+		if(q->state & Qclosed){
+			iunlock(q);
+			return -1;
+		}
+
 		/* we use an artificially high limit for kernel prints since anything
 		 * over the limit gets dropped
 		 */