Fix race condition and spelling error in metadata reader

2 separate fixes. See individual commit messages for more details.

1) Race condition revealed by http://roslyn.codeplex.com/workitem/299
2) Spelling error in rarely used pre-release API name




Not really as far as I can see. This simulates a reasonable use case (matching Roslyn if I understand correctly) where a PEReader is retained and ready to produce a MetadataReader on demand. We have to race on the allocation of the memory map which happens in GetMetadataReader.