* include/bits/shared_ptr_base.h (__shared_count(unique_ptr&&)): Check
	for nullptr (LWG 2415).
	* testsuite/20_util/shared_ptr/cons/unique_ptr_deleter.cc: Test
	construction from empty unique_ptr.
	* testsuite/20_util/shared_ptr/cons/43820_neg.cc: Adjust dg-error.
	* testsuite/20_util/shared_ptr/cons/void_neg.cc: Likewise.

From-SVN: r223170