isl_hash.c: reset the number of entries to 0 when growing the hash table

isl_hash.c: reset the number of entries to 0 when growing the hash table

The hash table is grown by creating a new table and adding the entries
of the old table to the new table.  Since the new table starts out as
being empty, the number of entries should be reset to 0.

Not only would the old code result in isl_union_map_n_map returing
the wrong value if the underlying hash table had ever been grown,
it would also result in the table subsequently being grown again
far too early.

Signed-off-by: Mythri Alle <mythri.alle@gmail.com>
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>