Crash when activating collisions after cell change

[gus]

To be fair, i have no idea why it works. I probably haven't looked deep enough in bullet source code. But it doesn't seems to break anything (this line should be necessary, but it seems that it is not).

[Zini]

Quoting myself:

This is somewhat unnerving. ...but removing a crash without fully understanding what went wrong is an invitation for further trouble.

Maybe I should not call it a fix, but a workaround. At some point this needs to be looked into. Ideally now. But considering that 0.12.0 is so hideously late already and that OpenMW isn't in productive use yet (e.g. people actually playing MW with it), a release with a workaround instead of a fix seems like a reasonable option.

[jbo_85]

This was already crashing in OpenMW 0.10.0.