fixed nasty bug with uninitialized game engine value

fixed nasty bug with uninitialized game engine value

This change fixed a nasty (although rarely occurring) bug caused by an
uninitialized game engine value for explosion handling (which can only
occur in the very first game frame of the very first game played after
starting the program).

This bug happens, for example, when doing "autotest rnd_helge_hafting"
or "autotest rnd_paul_e_collins_iii 013", while (as described above)
doing "autotest rnd_paul_e_collins_iii 012 013" works just fine (as
the game engine is correctly initialized by playing level 012 when it
comes to level 013, which contains that case of explosion in the first
frame that would else be played differently, causing a failed tape).