Workers: increase refcount when passing to thread
Workers: increase refcount when passing to thread

A temporary blockset can be collected before thread starts.

See https://travis-ci.org/npge/lua-npge/jobs/59856390#L1673

starius/lua-npge,npge/lua-npge,npge/lua-npge,starius/lua-npge,starius/lua-npge,npge/lua-npge