llvm-c-test avoid calling malloc(0)

Summary:
As explained in D63668, malloc(0) could return a null pointer. llvm-c-test does not handle this case correctly. Instead of calling malloc(0), avoid the operation altogether.

Authored By: andusy

Reviewers: hubert.reinterpretcast, xingxue, jasonliu, daltenty, cebowleratibm

Reviewed By: hubert.reinterpretcast

Subscribers: mehdi_amini, dexonsmith, jsji, llvm-commits

Tags: LLVM

Differential Revision: https://reviews.llvm.org/D63788

llvm-svn: 365144