Revert r342936 "Remove redundant null pointer check in operator delete"

A review for the change was opened in https://reviews.llvm.org/D52401
but the change was committed before being approved by any of the code
owners for libc++.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@342938 91177308-0d34-0410-b5e6-96231b3b80d8