Better error reporting from map::move_vehicle:

Add two separate checks for 1) empty displacement vector and 2) for displacement vector being too large.

Changed third check into an assertion as it is ensured by the splitting code above.

src/map.cpp

@@ -434,6 +434,14 @@ static bool sees_veh( const Creature &c, vehicle &veh, bool force_recalc )
 
 vehicle *map::move_vehicle( vehicle &veh, const tripoint &dp, const tileray &facing )
 {
+    if( dp == tripoint_zero ) {
+        debugmsg( "Empty displacement vector" );
+        return &veh;
+    } else if( abs( dp.x ) > 1 || abs( dp.y ) > 1 || abs( dp.z ) > 1 ) {
+        debugmsg( "Invalid displacement vector: %d, %d, %d", dp.x, dp.y, dp.z );
+        return &veh;
+    }
+
     // Split the movement into horizontal and vertical for easier processing
     if( dp.xy() != point_zero && dp.z != 0 ) {
         vehicle *const new_pointer = move_vehicle( veh, tripoint( dp.xy(), 0 ), facing );
@@ -449,14 +457,9 @@ vehicle *map::move_vehicle( vehicle &veh, const tripoint &dp, const tileray &fac
         result->is_falling = false;
         return result;
     }
-
     const bool vertical = dp.z != 0;
-    if( ( dp.x == 0 && dp.y == 0 && dp.z == 0 ) ||
-        ( abs( dp.x ) > 1 || abs( dp.y ) > 1 || abs( dp.z ) > 1 ) ||
-        ( vertical && ( dp.x != 0 || dp.y != 0 ) ) ) {
-        debugmsg( "move_vehicle called with %d,%d,%d displacement vector", dp.x, dp.y, dp.z );
-        return &veh;
-    }
+    // Ensured by the splitting above
+    assert( vertical == ( dp.xy() == point_zero ) );
 
     const int target_z = dp.z + veh.sm_pos.z;
     if( target_z < -OVERMAP_DEPTH || target_z > OVERMAP_HEIGHT ) {