[PATCH][widl] Fix uninitialized read in write_union_tfs

[Marvin]

Hi,

While running your changed tests, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?

Full results can be found at:
https://testbot.winehq.org/JobDetails.pl?Key=111835

Your paranoid android.


=== debian11 (build log) ===

error: corrupt patch at line 52
Task: Patch failed to apply

=== debian11 (build log) ===

error: corrupt patch at line 52
Task: Patch failed to apply