tsan: fix munmap interceptor

MetaMap::ResetRange/FreeRange rounds the range up to at least kMetaShadowSize.
This is requried for e.g. free(malloc(0)). However, munmap returns EINVAL
and do not unmap any memory when length arguments is equal to 0.
So don't free meta shadow in this case as well.

llvm-svn: 234145