Websocket: use pinfo pool and not packet one for the unmasked data tvb

As indicated in README.wmem, pinfo pool must be used for anything that might
end up being passed to add_new_data_source

Change-Id: I4a6396fc52e6c70c771390f502d58199c38c172a
Reviewed-on: https://code.wireshark.org/review/6144
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>