Fix memory leak in recv_skip

When the exception branch exits, the buf is leaked.

Reviewed by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: luozhengzheng <luo.zhengzheng@zte.com.cn>
Closes #5262

lib/libzfs/libzfs_sendrecv.c

@@ -3025,6 +3025,7 @@ recv_skip(libzfs_handle_t *hdl, int fd, boolean_t byteswap)
 		default:
 			zfs_error_aux(hdl, dgettext(TEXT_DOMAIN,
 			    "invalid record type"));
+			free(buf);
 			return (zfs_error(hdl, EZFS_BADSTREAM, errbuf));
 		}
 	}