[libcxx] Make std::basic_istream::get 0-terminate input array in case of error.

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