Fixed a critical bug in calculation of y_cross_p and y_cross_m in AGeoWinstonCone2D::DistToParabola. When |px|<<1 and |py|<<1 (i.e. photon is parallel to Z axis), py/px was meaningless.
Fixed a critical bug in calculation of y_cross_p and y_cross_m in AGeoWinstonCone2D::DistToParabola. When |px|<<1 and |py|<<1 (i.e. photon is parallel to Z axis), py/px was meaningless.

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