Add index bounds checks to negentropy ingester

[theAnuragMishra]

In the current state, it just trusts the inputarr.
This pr adds a check to verify the length of the input before tryiing to access arr.at(0) to check the type of the command.

Additionally, there might be some performance benefits of doing arr.get_array() beforehand and then direct index access instead of doing arr.at(i). Not sure how significant though.

[hoytech]

The size of the array is verified to be at least 2 in the runIngester loop (same with all commands), so the first arr[1] is safe. The NEG-OPEN branch then verifies at least 3 elements. And NEG-MSG uses .at() to get the message (which is bound checked).

So I don't think there actually is a missing bounds check anywhere.

However, your PR does clean up the code, and will provide a better error message in case of a malformed NEG-MSG command. The redundant verification of vals.size() is no big deal in practice, so I will merge. Thank you!