Reject zfs send -RI with nonexistent fromsnap

Right now, zfs send -I dataset@nonexistent dataset@existent fails, but
zfs send -RI dataset@nonexistent dataset@existent does not.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes openzfs#12574
Closes openzfs#12575

lib/libzfs/libzfs_sendrecv.c

@@ -2175,6 +2175,23 @@ zfs_send(zfs_handle_t *zhp, const char *fromsnap, const char *tosnap,
 		return (zfs_error(zhp->zfs_hdl, EZFS_NOENT, errbuf));
 	}
 
+	if (fromsnap) {
+		char full_fromsnap_name[ZFS_MAX_DATASET_NAME_LEN];
+		if (snprintf(full_fromsnap_name, sizeof (full_fromsnap_name),
+		    "%s@%s", zhp->zfs_name, fromsnap) >=
+		    sizeof (full_fromsnap_name)) {
+			err = EINVAL;
+			goto stderr_out;
+		}
+		zfs_handle_t *fromsnapn = zfs_open(zhp->zfs_hdl,
+		    full_fromsnap_name, ZFS_TYPE_SNAPSHOT);
+		if (fromsnapn == NULL) {
+			err = -1;
+			goto err_out;
+		}
+		zfs_close(fromsnapn);
+	}
+
 	if (flags->replicate || flags->doall || flags->props ||
 	    flags->holds || flags->backup) {
 		char full_tosnap_name[ZFS_MAX_DATASET_NAME_LEN];