Get per_buffer_config properly to avoid null

[ysirobs]

fixes #57

[xzbdmw]

Hello, thanks for pr, the problem is line 99 that should be changed to

        local b_config = config.getState("per_buffer_config")

that's the error if you don't set per_buffer_config, 50 is already a default value.

[ysirobs]

There we go :)