Use NULL instead of 0 for pointers.

The malloc will return a pointer to the allocated memory if successful,
otherwise a NULL pointer is returned.

usr.sbin/fstyp/fstyp.c

@@ -82,7 +82,7 @@ read_buf(FILE *fp, off_t off, size_t len)
 	}
 
 	buf = malloc(len);
-	if (buf == 0) {
+	if (buf == NULL) {
 		warn("cannot malloc %zd bytes of memory", len);
 		return (NULL);
 	}