Re: [PATCH] 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 While at it remove an outdated comment -- intptr_t has been used since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.