Fix behaviour of VelIntercept's oldvel parameter

Previously Vel would be reset only if the target was moving. This change
ensures that Vel is always reset, as seems to be the intent of the
oldvel parameter.

src/playsim/p_things.cpp

@@ -175,6 +175,7 @@ static void VelIntercept(AActor *targ, AActor *mobj, double speed, bool aimpitch
 {
 	if (targ == nullptr || mobj == nullptr)	return;
 
+	DVector3 prevel = mobj->Vel;
 	DVector3 aim = mobj->Vec3To(targ);
 	aim.Z += targ->Height / 2;
 
@@ -198,6 +199,10 @@ static void VelIntercept(AActor *targ, AActor *mobj, double speed, bool aimpitch
 			if (targ->Vel.X == 0 && targ->Vel.Y == 0)
 			{
 				InterceptDefaultAim(mobj, targ, aim, speed);
+				if (oldvel)
+				{
+					mobj->Vel = prevel;
+				}
 				return;
 			}
 		}
@@ -207,7 +212,6 @@ static void VelIntercept(AActor *targ, AActor *mobj, double speed, bool aimpitch
 		double a = g_acos(clamp(ydotx / targspeed / dist, -1.0, 1.0));
 		double multiplier = double(pr_leadtarget.Random2())*0.1 / 255 + 1.1;
 		double sinb = -clamp(targspeed*multiplier * g_sin(a) / speed, -1.0, 1.0);
-		DVector3 prevel = mobj->Vel;
 		// Use the cross product of two of the triangle's sides to get a
 		// rotation vector.
 		DVector3 rv(tvel ^ aim);
@@ -221,10 +225,6 @@ static void VelIntercept(AActor *targ, AActor *mobj, double speed, bool aimpitch
 		// And make the projectile follow that vector at the desired speed.
 		mobj->Vel = aimvec * (speed / dist);
 		mobj->AngleFromVel();
-		if (oldvel)
-		{
-			mobj->Vel = prevel;
-		}
 		if (aimpitch) // [MC] Ripped right out of A_FaceMovementDirection
 		{
 			const DVector2 velocity = mobj->Vel.XY();
@@ -235,6 +235,10 @@ static void VelIntercept(AActor *targ, AActor *mobj, double speed, bool aimpitch
 	{
 		InterceptDefaultAim(mobj, targ, aim, speed);
 	}
+	if (oldvel)
+	{
+		mobj->Vel = prevel;
+	}
 }
 
 DEFINE_ACTION_FUNCTION(AActor, VelIntercept)