`RevoluteJoint::angle()` occasionally incorrectly returns `NaN`

[chrisdunelm]

As far as I can see in my use-case, this is due to numeric imprecision which causes the i value of the ang_err quaternion to be marginally outside the range -1.0 to 1.0; which in turn causes i.asin() to return NaN.

A simple fix is to clamp the value passed to i.asin() to force it to the range -1.0 to 1.0