rigidbody: Don't run simulation if cache is baked

In rare cases this would allow the simulation to run before being
initialized (if cache is baked and reading cache fails after undo or
loading a file).

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54676 954f8c5b-7b00-dc11-b283-0030488c597c