Fix a bug about reading a zero-size map in the compact protocol implementation to lua

bug in lib/lua/TCompactProtocol.lua, code as follow:

function TCompactProtocol:readMapBegin()
local size = self:readVarint32()
local kvtype = 0
if size > 0 then
kvtype = self:readSignByte()
end
local ktype = self:getTType(libluabitwise.shiftr(kvtype, 4))
local vtype = self:getTType(kvtype)
return ktype, vtype, size
end

if the map's size is zero, it should‘t read another byte.