do not depend on signed integer overflow

Signed integer overflow is not defined in C, so do not depend on it.

This fixes a problem with GCC 4.4.0 and -O3 where the optimizer would
consider "consumed_bytes > consumed_bytes + bytes" as a constant
expression, and never execute the die()-call.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Acked-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/unpack-objects.c

@@ -83,7 +83,7 @@ static void use(int bytes)
 	offset += bytes;
 
 	/* make sure off_t is sufficiently large not to wrap */
-	if (consumed_bytes > consumed_bytes + bytes)
+	if (signed_add_overflows(consumed_bytes, bytes))
 		die("pack too large for current definition of off_t");
 	consumed_bytes += bytes;
 }