[libcxx] Do not assume the number of elements in a moved-from associative container

Reviewed as https://reviews.llvm.org/D57903.
Thanks to Andrey Maksimov for the patch.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@353955 91177308-0d34-0410-b5e6-96231b3b80d8