7571 non-present readonly numeric ZFS props do not have default value

Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>

usr/src/lib/libzfs/common/libzfs_dataset.c

@@ -22,7 +22,7 @@
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2013, Joyent, Inc. All rights reserved.
- * Copyright (c) 2011, 2015 by Delphix. All rights reserved.
+ * Copyright (c) 2011, 2016 by Delphix. All rights reserved.
  * Copyright (c) 2012 DEY Storage Systems, Inc.  All rights reserved.
  * Copyright (c) 2011-2012 Pawel Jakub Dawidek. All rights reserved.
  * Copyright (c) 2013 Martin Matuska. All rights reserved.
@@ -2037,6 +2037,7 @@ get_numeric_property(zfs_handle_t *zhp, zfs_prop_t prop, zprop_source_t *src,
 			if (zfs_prop_readonly(prop) &&
 			    *source != NULL && (*source)[0] == '\0') {
 				*source = NULL;
+				return (-1);
 			}
 			break;