kernel/fs: NULL is a valid buffer to pass to ioctl().

Another bug exposed by hrev52905. Discovered because Expander was
not displaying archive contents anymore.

src/system/kernel/fs/fd.cpp

@@ -915,7 +915,7 @@ _user_seek(int fd, off_t pos, int seekType)
 status_t
 _user_ioctl(int fd, uint32 op, void* buffer, size_t length)
 {
-	if (!IS_USER_ADDRESS(buffer))
+	if (buffer != NULL && !IS_USER_ADDRESS(buffer))
 		return B_BAD_ADDRESS;
 
 	TRACE(("user_ioctl: fd %d\n", fd));