Fix Lua library writeStructEnd() in TCompactProtocol

In 0.13.0, two changes were made to the Lua library's TCompactProtocol for writing structs: THRIFT-4992 (writeStructEnd) and THRIFT-5262 (writeStructBegin). Unfortunately these addressed the same issue, so the two changes together just recreated the original problem of corrupt field indices in nested structs.

Revert the change in THRIFT-4992 (and keep the one in THRIFT-5262).