Fix tuple getter in std unique pointer pretty-printer

Summary: Check case when _M_t child member is not present.

Reviewers: labath, tberghammer

Reviewed By: labath, tberghammer

Differential Revision: https://reviews.llvm.org/D47932
Patch by Aleksandr Urakov <aleksandr.urakov@jetbrains.com>.

llvm-svn: 334411