Fix minor issue in blsToExecChange message validator

[jtraglia]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

• Small fix in postAltairMsgID which used the wrong domain when calculating length; was using ValidSnappy in length calculation and using InvalidSnappy later. It doesn't make a difference though because both domains are the same length, four bytes. Just fixing to fix.
• Fix a typo in a comment about blob requests.
• The primary reason for this PR: check that blsChange.Message isn't nil before derefencing it. The gossip topic message validators, like this one, will recover from any panic. So Prysm won't crash, but it will print an annoying panic stack trace to the console.

[james-prysm]

cool thanks again justin, will let others know to take a look too if needed

[nisdas]

Nice Catch !