storage_proxy: Avoid potential use after move on schema_ptr
storage_proxy: Avoid potential use after move on schema_ptr

Paramter evaluation order is unspecified, so it's possible that the
move of 'schema' into lambda captures would happen before construction of
mutation.

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