Fix broken reading of SSH public keys from a file

[nnposter]

Loading of SSH public keys from a file via libssh2.read_publickey() fails because the routine does not preserve balanced Lua stack across operations performed on a luaL_Buffer object. Moving the code around rectifies the issue.

A minuscule optimization added by replacing lua_pushstring with lua_pushliteral.

The PR will be committed after September 6, 2024, unless concerns are raised.