Rapid-cue-switch crash bug mitigation

[xenimax]

Mitigation for #200

Prevent a crash when the player manages to arm the cue before their turn is up with the tip inside the cue ball, resulting in invalid location data. Ensures contents of relevant Sqrt functions is >= 0, and that stale location data isn't carried over on the first frame of the turn.

[CLAassistant]

All committers have signed the CLA.

[FairlySadPanda]

Apologies for the wait! Thanks for the bug fix. This won't make it into a formal release I think (I'm doing a 2.0 rewrite at the moment) but anything to help the master development branch's stability is welcomed.