fix: segfault when siphoning a vehicle with multiple tanks

[nphyx]

Summary

SUMMARY: Bugfixes "fixes segfault when attempting to siphon from vehicle with multiple tanks"

Purpose of change

When siphoning from a vehicle with more than one tank and the first tank is empty, during the check to auto-select the first tank with contents, a null pointer would be returned from pt.base.contents.legacy_front() ultimately resulting in a segfault.

Describe the solution

Remove the legacy_front() call and replace it with a proper !empty() check and only_item() call.

Describe alternatives you've considered

N/A

Testing

First start a fresh game, give yourself a jerrycan and a hose (and then remove the hose from the jerrycan ...), go find a vehicle with 2 fuel tanks, siphon the first, and then siphon the second. You should see a segfault.

Apply patch, repeat, no more segfault.

Additional context

This legacy_front() thing is extremely unsafe and may be the cause of several other crashes I'm seeing during playtesting. I put a debugmsg in it when it's about to return a nullptr, because this is almost always going to result in undefined behavior and this should help narrow down when this was the proximate cause.