Fixes potential bug in neighborsearching.

From put_in_list we first call new_i_nblist, then add some
j particles and then close_i_nblist

In new_i_nblist the number of i particles is typically
increased by one, that is nblist->nri is increased.

However, in case that there are no j particles, the nri
is not decreased again, which can lead to reading garbage
data from e.g. nblist->jjindex. In addition there is a
left-over variable (len) that computes the number of
j particles added, but this value is not used.

Change-Id: I38601a6865f6a4d879bec55246deded40943afb7