libstdc++: Fix invalid casts in unordered container merge functions

François pointed out that static_cast<__node_ptr>(&_M_before_begin) is
invalid, because _M_before_begin is only a node-base not a node.

Refactor the new merge overloads to only cast when we know we have a
valid node.

He also pointed out some optimizations to allow reusing hash codes that
might be cached in the node. The _M_src_hash_code function already has
the right logic to decide when a cached hash code can be reused by a
different _Hashtable object.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable.h (_Hashtable::_M_src_hash_code):
	Improve comments.
	(_Hashtable::_M_merge_unique(_Hashtable&)): Use pointer_traits
	to get before-begin pointer. Only use static_cast on valid
	nodes, not the before-begin pointer. Reuse a hash code cached in
	the node when possible.
	(_Hashtable::_M_merge_multi(_Hashtable&)): Likewise.

Reviewed-by: François Dumont <fdumont@gcc.gnu.org>