rts/M32Alloc: Don't attempt to unmap non-existent pages
The m32 allocator's pages list may contain NULLs in the case that the
page was flushed. Some munmap implementations (e.g. FreeBSD's) don't
like it if we pass them NULL. Don't do that.
Due to a large amount of spam we do not allow new users to create repositories, they are "external" users. If you are a new user and want to create a repository, for example for forking GHC, open a new issue on ghc/ghc using the "get-verified" issue template
The m32 allocator's pages list may contain NULLs in the case that the
page was flushed. Some munmap implementations (e.g. FreeBSD's) don't
like it if we pass them NULL. Don't do that.