dr_flac: Remove comparison that is always true

[dreamer]

Comparison is always true, because code above returns early when unsigned blockSize is 0. This invariant is asserted just before if-else chain.

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This is a follow up to #176 - to present an alternative fix. I also experimented with replacing these if-else's with a switch statement, but replacing this single comparison with an assert is less invasive change, and more readable.

[mackron]

Thanks. Merged to the dev branch.

[kcgen]

All three dev warning updates are passing, thank you @mackron!