Opm::WellState::init(): Handle shut wells

This is a minor bugfix to account for the presence of shut wells
(characterised by "ctrls[w]->current < 0").  The existing code would
lead to indexing outside the "ctrls" array in the context of a shut
well.
Merge remote-tracking branch 'upstream/master'
Merge pull request #117 from bska/master

Release preparation: Fix out-of-bounds indexing in context of shut wells