On error `getmaxyx()` may set the output params to -1.
We must check for this before treating the result as unsigned.

The `read()` function doesn't null terminate the `nwatch_str`, but
`strtol` requires it to be NULL-terminated.

In aspack decrypt function, there's a check to make sure that backbytes
doesn't exceed 57, because it is used as an index in init_array.
However, it is mathematically impossible.
So this commit removes the check.

Somehow forgot to save and commit final error handling check on the new
set_tls_client_certificate() function.
This change is needed to have Freshclam fail if you try to use the new
client certificate environment variables incorrectly.