Incorrect merkle validation in `process_deposit`

[gnattishness]

What is wrong?

process_deposit accepts deposits with an invalid merkle proof as valid

https://github.com/status-im/nim-beacon-chain/blob/257771d9af971621dc73b572c56d5e856ac75ad5/beacon_chain/spec/beaconstate.nim#L57-L70

How can it be fixed

1. process_deposit should fail if is_valid_merkle_branch() is false e.g. return false
2. is_valid_merkle_branch() should be called with depth=DEPOSIT_CONTRACT_TREE_DEPTH + 1 according to the spec (https://github.com/ethereum/eth2.0-specs/blob/v0.10.1/specs/phase0/beacon-chain.md#deposits)

How it was found

Discovered via beacon-fuzz (initial testing of deposit fuzzer).
Triggering case: nim-deposit-crash-5d4907f2962783d3806b93bfe1a5f4c808b1c3bf with the following beacon_states
Or the pre-processed input deposit_preprocessed_invalid_merkle.ssz can be directly passed to the nimbus harness nfuzz_deposit

[djrtwo]

Interesting, we have a case to cover an invalid merkle branch in the consensus tests here -- https://github.com/ethereum/eth2.0-specs/blob/7b76808a1c28dc44d449dee7619e301130066959/tests/core/pyspec/eth2spec/test/phase_0/block_processing/test_process_deposit.py#L228-L240

I'll see if I can figure out what the difference between this case is and why it's not adequately covered in our vectors

[tersec]

Feel free to re-open if this doesn't fix things.