Fix notice in honeypot_get_time_from_signed_timestamp()

This is probably the result of form cache entries, and on a typical site, these errors should go away within a few hours, once the cache entries get stale... I'm trying to think whether it's worth the effort to add this in (and cause every function call to have the extra μs worth of processing), considering that most sites that have already updated Honeypot should be good to go at this point, and those that haven't will still have old cache entries be flushed out soon after Honeypot is upgraded.

I'm going to mark this as closed—this is an issue that should go away for any site upgrading Honeypot now, and the number of sites that haven't yet upgraded and will is fairly small now.

I'm also getting this in production after recent upgrade, on multiple sites. All our caches have definitely been cleared multiple times. However, I am not seeing this error on every access to the registration form. I don't know if that's because honeypot is not active for me when I go to the reg form or what. Could this be caused by spammers submitting an outdated copy of the form?

Sounds good—also, if someone is attempting to forge or remove a timestamp value from the form submission, then that could throw a notice too, so I do see the value in this ticket.

Fixed in D7, switching over to D8.

Fixed in D8 as well. Since this and a couple other little issues affect a number of D7 sites, I'm going to go ahead and release 7.x-1.20 now, with these fixes.