sparse fix: Using plain integer as NULL pointer

Z_NULL is defined as 0, use a proper NULL pointer in its stead.
Signed-off-by: NRene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

archive-zip.c

@@ -160,7 +160,7 @@ static int write_zip_entry(const unsigned char *sha1,
 	void *buffer = NULL;
 	void *deflated = NULL;
 
-	crc = crc32(0, Z_NULL, 0);
+	crc = crc32(0, NULL, 0);
 
 	path = construct_path(base, baselen, filename, S_ISDIR(mode), &pathlen);
 	if (verbose)