PR libstdc++/51845
	* include/bits/hashtable.h
	(_Hashtable<>::erase(const_iterator, const_iterator)): Also update
	_M_buckets[__n_bkt] if __is_bucket_begin.

	* testsuite/23_containers/unordered_multimap/erase/51845-multimap.cc:
	New test.

From-SVN: r183300