[libcxx] Add assertion in deque::pop_back when popping from an empty deque

Also, add tests making sure that vector and deque both catch the problem
when assertions are enabled. Otherwise, deque would segfault and vector
would never terminate.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@348994 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed