bech32: add BIP-173 padding validation

[erickcestari]

@brunoerg has been doing differential fuzzing on CLightning and discovered an offer that crashes our differential fuzzing target in bitcoinfuzz. I found that CLightning accepts malformed offers containing invalid bech32 data that violates the BIP-173 specification, creating interoperability issues with other Lightning implementations (rust-lightning, lightning-kmp) that correctly reject such data.

BIP-173 says:

Re-arrange those bits into groups of 8 bits. Any incomplete group at the end MUST be 4 bits or less, MUST be all zeroes, and is discarded.

offer:

lno1qgqqvqtezcss8kkkkkkkkkk3srkklllllllvw03s33s333p444444444444pgqpq8

lightning-kmp check: https://github.com/ACINQ/bitcoin-kmp/blob/master/src/commonMain/kotlin/fr/acinq/bitcoin/Bech32.kt#L206

rust-lightning check: https://github.com/rust-bitcoin/rust-bech32/blob/master/src/primitives/iter.rs#L180

BOLT12 is missing a test case like this. Perhaps this could be a useful new test vector?

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