vehicle: fix sigfpe when acceleration is zero

It fixes rare case when all engines produce zero power due to no fuel or
exhausted character and target cruise speed is set less than current.

fixes CleverRaven#45473

src/vehicle_move.cpp

@@ -451,7 +451,7 @@ void vehicle::thrust( int thd, int z )
     //so 1000 = 100% and 453 = 45.3%
     int load;
     // Keep exact cruise control speed
-    if( cruise_on ) {
+    if( cruise_on && accel != 0 ) {
         int effective_cruise = std::min( cruise_velocity, max_vel );
         if( thd > 0 ) {
             vel_inc = std::min( vel_inc, effective_cruise - velocity );