[Sanitizer] Fix a possible write to freed memory in the wcrtomb interceptor

Summary:
r357240 added an interceptor for wctomb, which uses a temporary local
buffer to make sure we don't write to unallocated memory. This patch
applies the same technique to wcrtomb, and adds some additional tests
for this function.

Reviewers: vitalybuka, eugenis

Subscribers: kubamracek, delcypher, llvm-commits, #sanitizers

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D59984

llvm-svn: 357889