[Analyzer][MallocChecker] No warning for kfree of ZERO_SIZE_PTR.

Summary:
The kernel kmalloc function may return a constant value ZERO_SIZE_PTR
if a zero-sized block is allocated. This special value is allowed to
be passed to kfree and should produce no warning.

This is a simple version but should be no problem. The macro is always
detected independent of if this is a kernel source code or any other
code.

Reviewers: Szelethus, martong

Reviewed By: Szelethus, martong

Subscribers: rnkovacs, xazax.hun, baloghadamsoftware, szepet, a.sidorin, mikhail.ramalho, Szelethus, donat.nagy, dkrupp, gamesh411, Charusso, martong, ASDenysPetrov, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76830