Add a missing Invalid check to SubobjectDesignator::isOnePastEnd()

The class seems to have an invariant that Entries is non-empty if
Invalid is false.  It appears this method was previously private, and
all internal uses checked Invalid.  Now there is an external caller, so
check Invalid to avoid array OOB underflow.

Fixes PR20420.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213816 91177308-0d34-0410-b5e6-96231b3b80d8