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While at it remove an outdated comment -- intptr_t has been used since
ffe659f (parse-options: make some arguments optional, add callbacks),
committed in October 2007.

Signed-off-by: Rene Scharfe <[email protected]>
Acked-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rscharfe

builtin/fast-export.c

@@ -154,15 +154,14 @@ static void anonymize_path(struct strbuf *out, const char *path,
 	}
 }
 
-/* Since intptr_t is C99, we do not use it here */
-static inline uint32_t *mark_to_ptr(uint32_t mark)
+static inline void *mark_to_ptr(uint32_t mark)
 {
-	return ((uint32_t *)NULL) + mark;
+	return (void *)(uintptr_t)mark;
 }
 
 static inline uint32_t ptr_to_mark(void * mark)
 {
-	return (uint32_t *)mark - (uint32_t *)NULL;
+	return (uint32_t)(uintptr_t)mark;
 }
 
 static inline void mark_object(struct object *object, uint32_t mark)