Fix error propagation from lzc_send_redacted

Any error from lzc_send_redacted is overwritten by the error of
send_conclusion_record; skip writing the conclusion record if there
was an earlier error.

Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Philipp Riederer <philipp@riederer.email>
Closes #12766

lib/libzfs/libzfs_sendrecv.c

@@ -2542,7 +2542,7 @@ zfs_send_one(zfs_handle_t *zhp, const char *from, int fd, sendflags_t *flags,
 			    "progress thread exited nonzero")));
 	}
 
-	if (flags->props || flags->holds || flags->backup) {
+	if (err == 0 && (flags->props || flags->holds || flags->backup)) {
 		/* Write the final end record. */
 		err = send_conclusion_record(fd, NULL);
 		if (err != 0)