fix: Require prepare quorum on NEWVIEW M1 and reject incomplete CX proofs - #5138

Two small validation fixes. Behavior for normal blocks and valid proofs is unchanged.

NEWVIEW M1 prepare check

During view change, a NEWVIEW message can carry an M1 payload: the prepared block hash, plus the prepare signature and bitmap.

ProcessViewChangeMsg already requires that prepare bitmap to reach quorum. onNewView now does the same:

• payload must be at least ValidPayloadLength (32-byte hash plus signature bytes) before the hash is read
• prepare mask must meet quorum
• prepare signature must still verify against the hash

If those checks fail, the node stays in view change.

Incomplete CX receipts proofs

A cross-shard receipts proof is incomplete if required pointers are missing. Those fields are now treated as empty / invalid before they are used:

• nil receipt in the Receipts list → GetToShardID returns an error
• missing MerkleProof.BlockNum or header number → ContainsEmptyField is true
• nil Amount on CXReceipt.Copy → copy keeps amount as nil (big.Int.Set needs a non-nil source)

• No extra fork epoch is needed for rollout.