Fix crash when ps/source filtering.

We can't assume Msg here is going to be a msg record since
`logplex_utils:parse_msg` can sometimes return undefined for log lines
that don't match our pattern. Need to pull apart the record format
inside the match so we can fall down to the catch-all.

This should see the 500s we're sometimes seeing with filtered
tails. (the --ps or --source argument)