history: skip adding items if history is disabled

history: skip adding items if history is disabled

Previous to the migration to webkit2gtk, history items were not added
when either "history-max-items" was set to "0" or if a command was not
inserted by typing, but for example triggered by "au" commands. Right
now, though, this code is #ifdef'd out, so we'll always add items to the
history.

While we currently have no easy way of determining whether a command was
typed or not, we should still refuse adding history items whenever the
"history-max-items" variable is set to "0". As "au"-triggered commands
are not yet supported again, we do not even have to worry about this
case right now, but only later on when it is added. So we fix the issue
now by returning early when `history_max` is not set.