fast-export: avoid NULL pointer arithmetic

fast-export: avoid NULL pointer arithmetic

Clang 6 reports the following warning, which is turned into an error in a
DEVELOPER build:

	builtin/fast-export.c:162:28: error: performing pointer arithmetic on a null pointer has undefined behavior [-Werror,-Wnull-pointer-arithmetic]
		return ((uint32_t *)NULL) + mark;
		       ~~~~~~~~~~~~~~~~~~ ^
	1 error generated.

The compiler is correct, and the error message speaks for itself.  There
is no need for any undefined operation -- just cast mark to void * or
uint32_t after an intermediate cast to uintptr_t.  That encodes the
integer value into a pointer and later decodes it as intended.

While at it remove an outdated comment -- intptr_t has been used since
ffe659f94d (parse-options: make some arguments optional, add callbacks),
committed in October 2007.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>