compat/win32mmap.c: Fix some sparse warnings

Sparse issues two 'Using plain integer as NULL pointer' warnings
against the call to the CreateFileMapping() function. The warnings
relate to the second and sixth parameters, which both have pointer
type. In order to suppress the warnings, we simply pass the NULL
pointer, rather than '0', to those parameters in the function call.
Signed-off-by: NRamsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

compat/win32mmap.c

@@ -21,8 +21,8 @@ void *git_mmap(void *start, size_t length, int prot, int flags, int fd, off_t of
 	if (!(flags & MAP_PRIVATE))
 		die("Invalid usage of mmap when built with USE_WIN32_MMAP");
 
-	hmap = CreateFileMapping((HANDLE)_get_osfhandle(fd), 0, PAGE_WRITECOPY,
-		0, 0, 0);
+	hmap = CreateFileMapping((HANDLE)_get_osfhandle(fd), NULL,
+		PAGE_WRITECOPY, 0, 0, NULL);
 
 	if (!hmap)
 		return MAP_FAILED;