Free zone name when destroying malloc zone

Summary:
The darwin interceptor for malloc_destroy_zone manually frees the
zone struct, but does not free the name component. Make sure to
free the name if it has been set.

Reviewers: kubamracek, alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31983

llvm-svn: 300195