fix(bits): guard BitArray against malformed Bits/Elems from peer proto

[FletcherMan]

A peer can send a consensus message (NewValidBlock/ProposalPOL/VoteSetBits) whose BitArray carries Bits>0 but no Elems. FromProto writes Bits unconditionally but only writes Elems when non-empty, so the decoded array is internally inconsistent. getTrueIndices then indexes Elems[-1] and panics with "index out of range [-1]", crashing the node from the catchup gossip path (reactor.go gossipDataForCatchup -> PickRandom -> getTrueIndices).

Minimal, surgical hotfix on the three bit_array.go sites that index Elems based on Bits without checking len(Elems):

• getTrueIndices: treat a Bits/Elems-inconsistent array as having no set bits
• setIndex: bounds-check i/64 against len(Elems)
• IsFull: avoid Elems[:len-1] panic on empty Elems

Does not change FromProto or message ValidateBasic; a fuller validation-based fix (ported from CometBFT v0.40.0) is tracked separately.

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PR checklist

• Tests written/updated, or no tests needed
• CHANGELOG_PENDING.md updated, or no changelog entry needed
• Updated relevant documentation (docs/) and code comments, or no
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