ND2: reset series count before allocating offset array

This prevents extra memory from being used if the series count was too
high.  See http://lists.openmicroscopy.org.uk/pipermail/ome-users/2014-September/004699.html
Merge pull request #1398 from melissalinkert/rebased/dev_5_0/nd2-memory

ND2: reset series count before allocating offset array (rebased onto dev_5_0)