transport/server: Fix CQL Snappy compression failure
transport/server: Fix CQL Snappy compression failure

The snappy_compress() function expects the "compressed_length" parameter
to contain the actual output buffer length but now we're passing random
garbage from the stack.

Fixes #1711
Message-Id: <e1910ffb6a04aa33d2ca4286f002d7b922e6c30b@scylladb.com>

duarten/scylla,avikivity/scylla,scylladb/scylla,raphaelsc/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,raphaelsc/scylla,avikivity/scylla,raphaelsc/scylla,duarten/scylla