zfs send does not handle invalid input gracefully

Due to some changes introduced in 30af21b 'zfs send' can crash when
provided with invalid inputs: this change attempts to add more checks
to the affected code paths.

Reviewed-by: Attila Fülöp <attila@fueloep.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #9001

lib/libzfs/libzfs_sendrecv.c

@@ -2422,6 +2422,10 @@ zfs_send(zfs_handle_t *zhp, const char *fromsnap, const char *tosnap,
 		}
 		zfs_handle_t *tosnap = zfs_open(zhp->zfs_hdl,
 		    full_tosnap_name, ZFS_TYPE_SNAPSHOT);
+		if (tosnap == NULL) {
+			err = -1;
+			goto err_out;
+		}
 		err = send_prelim_records(tosnap, fromsnap, outfd,
 		    flags->replicate || flags->props || flags->holds,
 		    flags->replicate, flags->verbosity > 0, flags->dryrun,
@@ -2707,6 +2711,8 @@ zfs_send_one(zfs_handle_t *zhp, const char *from, int fd, sendflags_t *flags,
 	if (from != NULL && strchr(from, '@')) {
 		zfs_handle_t *from_zhp = zfs_open(hdl, from,
 		    ZFS_TYPE_DATASET);
+		if (from_zhp == NULL)
+			return (-1);
 		if (!snapshot_is_before(from_zhp, zhp)) {
 			zfs_close(from_zhp);
 			zfs_error_aux(hdl, dgettext(TEXT_DOMAIN,