fix usage of the ACTNUM keyword exported by EclipseGrid

it turns out that EclipseGrid::exportACTNUM() produces a vector of
zero length instead of an all-1 one of the size of the logically
Cartesian grid if ACTNUM is not set.

Strangely enough, this worked if libstdc++'s debug mode was not
enabled. (maybe because dune-cornerpoint does not consider ACTNUM?)
fix incorrect C-pointer ACTNUM handling

these C-style arrays can be NULL!
Merge pull request #102 from andlaus/correct_actnum_usage

fix usage of the ACTNUM keyword exported by EclipseGrid