Bug in btCollisionDispatcherMt

[samojordan]

btCollisionDispatcherMt::dispatchAllCollisionPairs() has a bug which triggers when
a manifold is created and released during a single dispatch. The fix is to move
the loop that updates the m_index1a indices to run before the loop which releases
the manifolds. We don't have a minimal example for reproduction, we encountered this
in a large application. We have our own clone of bullet and have fixed it there so for
us this is done, but would be nice if someone converts this report into a proper
bugreport/PR such that all other bullet users can benefit.