Fix invalid avatar when we have old memorial

[rtxyd]

Summary

Bugfixes "Fix invalid avatar when we have old memorials"

Purpose of change

Fix Strange: player is already assigned to mission 1 through 10
#67277

Describe the solution

Add a check to this function.

...(loading data)
if( u.getID() != character_id( -1 ) ){
...(do something)
}

Load game if there's an old memorial that don't have event_type::game_avatar_new, or event_type::game_start,
will cause "strange: player is already assigned to mission 1", actually, this is because, the player ID hasn't been set by game,
and the value is -1, and when we trigger event_type::game_avatar_new, game will assign misson to player, but at this time the target player id of the mission is also -1, so we got this bug.

the u ID is the player ID in current world, so if we load an existed save, this should not happen,
and when we load memorial, the same function will be invoke, so it may happen at this situation.

if the player ID don't set yet, it's -1, this situation only occurs when we have an old memorial without event_type::game_avatar_new or event_type::start_game,
and if the player ID set, then this function need to be done for old savefiles.

Describe alternatives you've considered

Maybe, we can optimize event function.

Testing

try file from the issue not trigger the case.

Steps to reproduce:

1. Download memorial.zip, decompress and put memorial folder in game folder
2. Launch game, start a new game: New Game -> Play Now!
3. Bug

Additional context

This is how the bug works.

Tell me if the code's anything wrong.