Fix garage refuel handling of bad/old state data

[jaj22]

What type of PR is this.

1. Bug
2. Change
3. Enhancement

What have you changed and why?

1. If you put a fuel vehicle into the garage with ACE loaded, it'll write the vanilla fuel cargo as -1. This was then used for calculations of available fuel, causing ACE + non-ACE garaged tankers to report 0 fuel total. It also broke the actual refuel function. Fixed with a scattering of 0 max.

2. At some point the garage was capable of writing nil as the ACE fuel cargo. If then used with ACE, the refuel function would do its crackheaded infinite loop and then fail to refuel anything. Fixed with a nil check.

Also sanitized some related code. Should really rework the refuel function to be less dangerous but maybe later.

Please specify which Issue this PR Resolves.

closes #XXXX

Please verify the following and ensure all checks are completed.

1. Have you loaded the mission in LAN host?
2. Have you loaded the mission on a dedicated server?

Is further testing or are further changes required?

1. No
2. Yes (Please provide further detail below.)