PLCgen test model "tuples" does not compile

CIF model: tuples.cif

Generated PLC code: DB.db

MAIN.scl

TupleStruct.udt TupleStruct_1.udt TupleStruct_2.udt TupleStruct_3.udt TupleStruct_4.udt TupleStruct_5.udt TupleStruct_6.udt TupleStruct_7.udt TupleStruct_8.udt TupleStruct_9.udt TupleStruct_10.udt TupleStruct_11.udt TupleStruct_12.udt TupleStruct_13.udt TupleStruct_14.udt TupleStruct_15.udt

Compiling the PLC code in this tests results in a single error. This may indicate a typo, as most tuple structs seem to compile correctly.

Addresses #679