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



This is the same as gh-1351 but rebased onto dev_5_0.

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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 and the file in ```nd2/christophe/```.