get-tar-commit-id: check write_in_full() return against 0

We ask to write 41 bytes and make sure that the return value
is at least 41. This is the same "dangerous" pattern that
was fixed in the prior commit (wherein a negative return
value is promoted to unsigned), though it is not dangerous
here because our "41" is a constant, not an unsigned
variable.

But we should convert it anyway to avoid modeling a
dangerous construct.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/get-tar-commit-id.c

@@ -33,8 +33,7 @@ int cmd_get_tar_commit_id(int argc, const char **argv, const char *prefix)
 	if (!skip_prefix(content, "52 comment=", &comment))
 		return 1;
 
-	n = write_in_full(1, comment, 41);
-	if (n < 41)
+	if (write_in_full(1, comment, 41) < 0)
 		die_errno("git get-tar-commit-id: write error");
 
 	return 0;