libbe: fix be_mounted_at() with props after bootonce

propinfo.bootonce was filled with garbage, leading to a segfault later
during prop building.  Initialize it to NULL.

MFC after:	3 days

lib/libbe/be_access.c

@@ -219,6 +219,7 @@ be_mounted_at(libbe_handle_t *lbh, const char *path, nvlist_t *details)
 			propinfo.lbh = lbh;
 			propinfo.list = details;
 			propinfo.single_object = false;
+			propinfo.bootonce = NULL;
 			prop_list_builder_cb(root_hdl, &propinfo);
 			zfs_close(root_hdl);
 		}