reftable: fix resource warning

This would trigger in the unlikely event that we are compacting, and
the next available file handle is 0.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/stack.c

@@ -877,7 +877,7 @@ static int stack_compact_range(struct reftable_stack *st, int first, int last,
 	struct strbuf new_table_path = STRBUF_INIT;
 	int err = 0;
 	int have_lock = 0;
-	int lock_file_fd = 0;
+	int lock_file_fd = -1;
 	int compact_count = last - first + 1;
 	char **listp = NULL;
 	char **delete_on_success =
@@ -911,7 +911,7 @@ static int stack_compact_range(struct reftable_stack *st, int first, int last,
 	}
 	/* Don't want to write to the lock for now.  */
 	close(lock_file_fd);
-	lock_file_fd = 0;
+	lock_file_fd = -1;
 
 	have_lock = 1;
 	err = stack_uptodate(st);
@@ -1013,7 +1013,7 @@ static int stack_compact_range(struct reftable_stack *st, int first, int last,
 		goto done;
 	}
 	err = close(lock_file_fd);
-	lock_file_fd = 0;
+	lock_file_fd = -1;
 	if (err < 0) {
 		err = REFTABLE_IO_ERROR;
 		unlink(new_table_path.buf);
@@ -1050,9 +1050,9 @@ done:
 		listp++;
 	}
 	free_names(subtable_locks);
-	if (lock_file_fd > 0) {
+	if (lock_file_fd >= 0) {
 		close(lock_file_fd);
-		lock_file_fd = 0;
+		lock_file_fd = -1;
 	}
 	if (have_lock) {
 		unlink(lock_file_name.buf);