backend: Remove some void/unsafe initialization

[dkorpel]

Getting rid of void initialization because it's bug prone: #21099

There's more, but I picked these ones first because they are:

• redundant (because they were initialized immediately afterwards)
• __gshared (so they get 0 initializated anyway)
• small / not performance critical

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[thewilsonator]

Apart from the void initialised union members, this looks good.