reftable: handle block-writer initialization errors

2d5dbb37b2 (reftable/block: handle allocation failures, 2024-10-02)
taught `writer_reinit_block_writer()` to report initialization failures
and updated its callers, but `reftable_writer_new()` continued to ignore
the return value.

Consequently, the constructor could report success after block-writer
initialization had failed. Propagate the error and release the
constructor's allocations instead of returning an unusable writer.

Pointed out by GPT-5.6 Sol and Claude Opus 4.8.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/writer.c

@@ -150,6 +150,7 @@ int reftable_writer_new(struct reftable_writer **out,
 {
 	struct reftable_write_options opts = {0};
 	struct reftable_writer *wp;
+	int err;
 
 	if (_opts)
 		opts = *_opts;
@@ -177,7 +178,12 @@ int reftable_writer_new(struct reftable_writer **out,
 	wp->opts = opts;
 	wp->hash_id = hash_id;
 	wp->flush = flush_func;
-	writer_reinit_block_writer(wp, REFTABLE_BLOCK_TYPE_REF);
+	err = writer_reinit_block_writer(wp, REFTABLE_BLOCK_TYPE_REF);
+	if (err < 0) {
+		reftable_free(wp->block);
+		reftable_free(wp);
+		return err;
+	}
 
 	*out = wp;