Remove redundant null pointer check in operator delete

Remove redundant null pointer check in operator delete

C89 4.10.3.2 The free function
C99 7.20.3.2 The free function
C11 7.22.3.3 The free function

    If ptr is a null pointer, no action shall occur.

_aligned_free on MSDN:

    If memblock is a NULL pointer, this function simply performs no actions.

Reviewers: EricWF, mclow.lists

Subscribers: christof, ldionne, cfe-commits, libcxx-commits

Differential Revision: https://reviews.llvm.org/D52401

llvm-svn: 342936