correct indirection of nil.  (steve stallion)

also, return exit-status 1 in case of wait failure, and document
the conversion from wait message to unix wait status which wasn't
otherwise instantly obvious.

Reference: /n/atom/patch/applied2013/sshwait2
Date: Wed Oct  2 16:22:49 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/ssh2/sshsession.c	Wed Oct  2 16:19:04 2013
+++ /sys/src/cmd/ssh2/sshsession.c	Wed Oct  2 16:19:04 2013
@@ -415,16 +415,20 @@
 		while (1) {
 			fprint(errfd, "waiting for child %d\n", cmdpid);
 			w = wait();
-			fprint(errfd, "child %d passed\n", w->pid);
-			if (w->pid == cmdpid || w->pid == -1)
+			if (w == nil || w->pid == cmdpid)
 				break;
 			free(w);
 		}
-		if (w->pid == -1)
+		if (w == nil){
 			fprint(errfd, "wait failed: %r\n");
-		else
-			fprint(reqfd, "exit-status %d", w->msg[0]);
-		free(w);
+			fprint(reqfd, "exit-status 1");
+		}
+		else {
+			fprint(errfd, "child %d exit-status %d\n", w->pid, *w->msg);
+			/* empty msg ≡ exit(0); non-empty ≡ exit(1) */
+			fprint(reqfd, "exit-status %d", *w->msg != 0);
+			free(w);
+		}
 		if (slfd > 0)
 			fprint(slfd, "closing ssh session for %s\n", uname);
 		else