riff: fix clippy warnings, fix overflow with u16 for frames_per_block calculation #534

Job	Run time
Test x86_64-unknown-linux-gnu on ubuntu-latest	56s
Rustfmt	10s
Test aarch64-apple-darwin on macos-latest	45s
Check ubuntu-latest	22s
Test x86_64-pc-windows-msvc on windows-latest	2m 41s
Clippy ubuntu-latest	27s
Check macos-latest	25s
Test powerpc-unknown-linux-gnu on ubuntu-latest	1m 44s
Clippy macos-latest	32s
Check windows-latest	1m 48s
Clippy windows-latest	1m 35s
	11m 25s

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