Fix: Uninitialized variable in Income/Expence summary

[hallonsoda79]

Motivation / Problem

While browsing in the code I found an uninitialized variable in the Income/Expence summary that could lead to wrongly calculated yearly total sum.

Description

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[nielsmh]

The OverflowSafeInt type (of which Money is a specialization) has a default constructor that initializes it to 0. I don't think this is actually a bug.

[frosch123]

Money is a real class, no POD integer. It has a default constructor.