Do not decrement battle_counter for respawning Pkunk

 * As of commit dde9ab864b the decrement of battle_counter
   in ship_death() was unguarded - previously, it had been
   conditional on a non-obvious property, namely ship mass
   being within the normal limits:

	if (ShipPtr->mass_points <= MAX_SHIP_MASS)

   Both retreating ships and respawning Pkunk apparently have
   their mass_points set above MAX_SHIP_MASS, so removing this
   conditional had the unintended consequence of causing Pkunk
   respawns to be counted as dead ships! We now just check for
   respawning Pkunk.