Duke3D: savemapstate can overwrite the wrong map slot

If one executes the command savemapstate immediately after the userdef struct members volume_number and level_number are updated, the command will store the state of the currently loaded map in the slot pointed to by level_number and volume_number, instead of the proper map slot.

Subsequently executing loadmapstate on said slot will then result in the game loading a copy of the previous map in the new slot.

There are several ways in which this can happen. Examples:

Simply executing savemapstate after startlevel:

onevent EVENT_ENTERLEVEL
    loadmapstate
endevent

onevent EVENT_ALTFIRE
    startlevel 1 5
    savemapstate // E2L6 is now overridden
endevent

Executing savemapstate inside EVENT_ENDLEVELSCREEN

onevent EVENT_ENTERLEVEL
    loadmapstate
endevent

onevent EVENT_ENDLEVELSCREEN
    savemapstate // will override the next map always
endevent

During testing of WGR2 we have also encountered this problem in a case where the mapstate was saved on the same tick where the player exited the map using sector lotag -1.