PR libstdc++/68197 fail on negative iword/pword indices

PR libstdc++/68197 fail on negative iword/pword indices

The suggested resolution of LWG 3083 is to make invalid indices
undefined, but we can fairly easily check for them and treat them as
errors in the same way as allocation failure. This avoids a segfault or
worse, setting an error flag on the stream instead.

	PR libstdc++/68197
	* include/bits/ios_base.h (ios_base::iword, ios_base::pword): Cast
	indices to unsigned.
	* src/c++11/ios.cc (ios_base::_M_grow_words): Treat negative indices
	as failure. Refactor error handling.
	* testsuite/27_io/ios_base/storage/68197.cc: New.

From-SVN: r259854