When be_activate() turns on a new boot environment, it always deactivates

the current one first. And if it fails to do so, it abandons activation.
However, with the new bootonce feature, there is a legitimate case when
a pool doesn't have "bootfs" property set. Check for this case before
calling be_deactivate().

Reviewed by:	kevans

lib/libbe/be.c

@@ -1263,7 +1263,8 @@ be_activate(libbe_handle_t *lbh, const char *bootenv, bool temporary)
 		return (lzbe_set_boot_device(
 		    zpool_get_name(lbh->active_phandle), lzbe_add, be_path));
 	} else {
-		if (be_deactivate(lbh, lbh->bootfs, false) != 0)
+		if (strncmp(lbh->bootfs, "-", 1) != 0 &&
+		    be_deactivate(lbh, lbh->bootfs, false) != 0)
 			return (-1);
 
 		/* Obtain bootenv zpool */