Avoid undefined behavior

[AlanGriffiths]

We don't guarantee that XKBMapper::map_event() sets last_device_id before we use it in XKBMapper::update_modifier() and valgrind complains.

[codecov]

Codecov Report

Merging #3055 (850d34b) into main (076ebfd) will decrease coverage by 0.01%.
Report is 2 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3055      +/-   ##
==========================================
- Coverage   77.70%   77.69%   -0.01%     
==========================================
  Files        1056     1056              
  Lines       73409    73409              
==========================================
- Hits        57041    57037       -4     
- Misses      16368    16372       +4     

Files	Coverage Δ
src/common/input/xkb_mapper.cpp	78.83% <100.00%> (ø)

... and 4 files with indirect coverage changes

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[Saviq]

Yups.