src/openssl.c: save top before calling luaL_buffinit

[daurnimator]

No description provided.

[Zash]

For future reference, this fixes a bad argument #2 to 'final' (string expected, got light userdata) error when you require"openssl.cipher".new("AES-128-GCM"):encrypt(key, iv):final("x") on Lua 5.4 after lua/lua@cf23a93

I observe that this is a year old and not yet in a release and is affecting users in the wild.