If append_archive fails while writing an archive header, output the error

message from the archive being written (not the message from the archive
being read, where no error has occurred).

MFC after:	3 days

usr.bin/tar/write.c

@@ -488,7 +488,7 @@ append_archive(struct bsdtar *bsdtar, struct archive *a, const char *filename)
 		/* XXX handle/report errors XXX */
 		if (archive_write_header(a, in_entry)) {
 			bsdtar_warnc(bsdtar, 0, "%s",
-			    archive_error_string(ina));
+			    archive_error_string(a));
 			bsdtar->return_value = 1;
 			return (-1);
 		}