Change: Don't save industry history if cargo slot isn't used.

[PeterN]

Motivation / Problem

Industries have 16 slots which may produce cargo, and we save two months of production/transported history for each slot for each industry.

In many cases (dependent on NewGRFs), 14 of the 16 slots won't actually ever be used as cargo slot is CT_INVALID.

Description

Skip saving history if the slot is CT_INVALID by setting the saved length to 0.

This avoids saving history of 16 slots per industry when in many cases (NewGRF dependent) only a couple are used.

Does not need a saveload bump as the loading code is already told how much data there is.

Limitations

As savegames are compressed, this doesn't particularly save much space... on a 4096x4096 map with high industries the resultant file size difference is negli:ble.

Vanilla: 24,766,936 bytes
Patched: 24,760,648 bytes

This may be of more benefit with #10541, but... no idea.

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

[PeterN]

Oops, this should use IsValidCargoId() :-)

[ldpl]

Did a bit of testing, seems to be a slight improvement overall. Though uncompressed size doesn't add up, I'd expect about 32 * 2 * 14 = 896 bytes per industry reduction based on the description of this PR, but got 119 instead.

Timing patch:
savetiming.txt
Command: ./openttd -G 1 -g -v null:ticks=55000 && ls -lsa ~/.openttd/save/autosave/exit.sav