It covers the cases when the sentry object returns false and when an exception
was thrown. Corresponding standard paragraph is C++14 [istream.unformatted]p9:
[...] In any case, if n is greater than zero it then stores a null
character into the next successive location of the array.
rdar://problem/35566567
Reviewers: EricWF, mclow.lists
Reviewed By: mclow.lists
Subscribers: cfe-commits
Differential Revision: https://reviews.llvm.org/D40677
llvm-svn: 322326