fuzzy zero assert when creating certain btGeneric6DofConstraints

[pfanne]

In btGeneric6DofConstraint.cpp in line 374:
When axis2 in frameB and axis0 in frameA align at any point in time it results in a Zero-Vector which will later be normalized which results in a fuzzy zero assert.
This is mainly an issue when creating a new constraint where initial positions are orthogonal to each other, or a constraint that has no influence on the orientation of the objects.
This can be really annoying when running in debug mode and can potentially create unexpected behavior in release mode

Some code to reproduce the issue:
http://pastebin.com/psHNgN6Q