libstdc++: Initialize all members of hashtable local iterators

libstdc++: Initialize all members of hashtable local iterators

Currently the _M_bucket members are left uninitialized for
default-initialized local iterators, and then copy construction copies
indeterminate values. We should just ensure they're initialized on
construction.

Setting them to zero makes default-initialization consistent with
value-initialization and avoids indeterminate values.

For the _Local_iterator_base<..., false> specialization we preserve the
existing behaviour of setting _M_bucket_count to -1 in the default
constructor, as a sentinel value to indicate there's no hash object
present.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable_policy.h (_Local_iterator_base): Use
	default member-initializers.