snep: Fix SNEP fragmentation

snep: Fix SNEP fragmentation

The intial request is kept through the snep_data and
snep_core_process_request no longer relies on the last read frame to
build an answer but only on the snep_data pointer.
Without this fix, each fragment from a fragmented PUT was handled as
a complete SNEP frame (header included) which caused process_request
to get completely confused about how to handle fragments.
Now that snep_core_read is fixed, the fragmentation SNEP unit test can
be fixed as well.