Fix Pryaxis/TShock#3073

[sgkoishi]

This is a vanilla bug.

Some projectiles move in a way that depends on the position of the player, and they try to compute their 'direction' relative to the player, for example, player.Center - projectile.Center.

The Counterweight appears to use player.Center as its default position, and it also calculates the direction (rotating), which is { X: 0, Y: 0 }. Zero vector can't be normalized and attempt to normalize it will result in { X: NaN, Y: NaN } - so the velocity and position are messed up because NaN values were added to them.

See also:

• Terraria.Player.Counterweight()
• Terraria.Projectile.HandleMovement() && aiStyle == 99