fix(gossip): reject EpochSlots with null BitVec bits in sanitize

[dymchenkko]

Implementations

• Null check in Uncompressed.sanitize rejects messages with null bits before they enter the gossip table
• Made BitVec.deinit null-safe while at it

Ramifications

• Peer sends one bad EpochSlots packet -> passes sanitize -> sits in table -> panics on eviction. Delayed crash makes it hard to trace back to the incoming message.

Tests

• Test now verifies sanitize rejects null bits and deinit survives them

Closes #1564