Be defensive against a reentrant std::function::operator=(nullptr_t), in case
the held function object has a non-trivial destructor. Destroying the function
object in-place can lead to the destructor being called twice.
Patch by Duncan P. N. Exon Smith. C++03 support by Volodymyr Sapsai.
rdar://problem/32836603
Reviewers: EricWF, mclow.lists
Reviewed By: mclow.lists
Subscribers: cfe-commits, arphaman
Differential Revision: https://reviews.llvm.org/D34331
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@330885 91177308-0d34-0410-b5e6-96231b3b80d8
