net: fix error reported on job exit and full residual

This is expected, when one end drops off.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/engines/net.c b/engines/net.c
index 1d89db1..61bdfdd 100644
--- a/engines/net.c
+++ b/engines/net.c
@@ -515,11 +515,13 @@
 		ret = 0;	/* must be a SYNC */
 
 	if (ret != (int) io_u->xfer_buflen) {
-		if (ret >= 0) {
+		if (ret > 0) {
 			io_u->resid = io_u->xfer_buflen - ret;
 			io_u->error = 0;
 			return FIO_Q_COMPLETED;
-		} else {
+		} else if (!ret)
+			return FIO_Q_BUSY;
+		else {
 			int err = errno;
 
 			if (ddir == DDIR_WRITE && err == EMSGSIZE)