Fix potential NULL pointer dereference in zfsdle_vdev_online()

Coverity complained about this.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Chunwei Chen <david.chen@nutanix.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes openzfs#13903

cmd/zed/agents/zfs_mod.c

@@ -965,7 +965,7 @@ zfsdle_vdev_online(zpool_handle_t *zhp, void *data)
 	nvlist_t *tgt;
 	int error;
 
-	char *tmp_devname, devname[MAXPATHLEN];
+	char *tmp_devname, devname[MAXPATHLEN] = "";
 	uint64_t guid;
 
 	if (nvlist_lookup_uint64(udev_nvl, ZFS_EV_VDEV_GUID, &guid) == 0) {