segment_names variable in xen layer potential backtrace

[atcuno]

The segment_names variable is used after the loop but not initialized before, which would have a crash if the loop doesn't set it.

https://github.com/volatilityfoundation/volatility3/blob/develop/volatility3/framework/layers/xen.py#L68

it needs to be initialized to None before the loop to prevent this.

Can you fix this real quick @gcmoreira ?