PrismaticJoint.prototype.getReactionForce returns NaN about planck.js

[weilichuang]

var result = Vec2.zero();
result.x = inv_dt * (this.m_impulse.x * this.m_perp.x + (this.m_motorImpulse + this.m_impulse.z) * this.m_axis.x);
result.y = inv_dt * (this.m_impulse.x * this.m_perp.y + (this.m_motorImpulse + this.m_impulse.z) * this.m_axis.y);
return result;

[shakiba]

I fixed this. Thanks for reporting, please feel free to reopen if needed.