Prevent discharging more power from battery with nearly 0% power

[AngelicosPhosphoros]

Summary

SUMMARY: Bugfixes "Prevent discharging too much power from nearly empty batteries"

Purpose of change

Consider the situation when charging from large storage battery (80 MJ) with 10 kJ remaining. It would have 0% power level (10*100 / 80_000) so our "prev_charge_level" would -800 and we would lower amount by 810.

Essentially free energy when it shouldn't be so I ensure that we cannot charge more power than remaining.

Describe the solution

Just ensure that we never try to lower our power amount more than we have.

Describe alternatives you've considered

Testing

Only run autotests because change is trivial.

Additional context

[Coolthulhu]

Tiny nit: functional style assignments (variables that could be consted with no changes to the code) are preferred to "procedural" ones.

[AngelicosPhosphoros]

Tiny nit: functional style assignments (variables that could be consted with no changes to the code) are preferred to "procedural" ones.

Here it was already like that and I found putting everything into one expression too bulky in that case