Fix uninitialized variable warning in vdev_prop_get()

Update vdev_prop_get_objid() to set objid on error as the comment
in vdev_prop_get() describes.

    "objid is set to 0 when absent and the few cases that call
    zap_lookup directly guard against this below."

This resolves the following possible uninitialized variable warning.

    module/zfs/vdev.c: In function ‘vdev_prop_get’:
    module/zfs/vdev.c:6913:12: error: ‘objid’ may be used uninitialized
    in this function [-Werror=maybe-uninitialized]

Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #18616

module/zfs/vdev.c

@@ -460,6 +460,7 @@ vdev_prop_get_objid(vdev_t *vd, uint64_t *objid)
 	} else if (vd->vdev_leaf_zap != 0) {
 		*objid = vd->vdev_leaf_zap;
 	} else {
+		*objid = 0;
 		return (EINVAL);
 	}
 
@@ -6444,7 +6445,7 @@ vdev_prop_get(vdev_t *vd, nvlist_t *innvl, nvlist_t *outnvl)
 	spa_t *spa = vd->vdev_spa;
 	objset_t *mos = spa->spa_meta_objset;
 	int err = 0;
-	uint64_t objid;
+	uint64_t objid = 0;
 	uint64_t vdev_guid;
 	nvpair_t *elem = NULL;
 	nvlist_t *nvprops = NULL;