Stop segfaulting on unmount error case

After interrupting ZTS runs that errored out, I found that
"zpool export testpool2" was segfaulting.

This seems unnecessary.

Reviewed-by: szubersk <szuberskidamian@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes openzfs#12804

lib/libzfs/libzfs_mount.c

@@ -595,9 +595,13 @@ unmount_one(zfs_handle_t *zhp, const char *mountpoint, int flags)
 		default:
 			libzfs_err = EZFS_UMOUNTFAILED;
 		}
-		return (zfs_error_fmt(zhp->zfs_hdl, libzfs_err,
-		    dgettext(TEXT_DOMAIN, "cannot unmount '%s'"),
-		    mountpoint));
+		if (zhp) {
+			return (zfs_error_fmt(zhp->zfs_hdl, libzfs_err,
+			    dgettext(TEXT_DOMAIN, "cannot unmount '%s'"),
+			    mountpoint));
+		} else {
+			return (-1);
+		}
 	}
 
 	return (0);