Guard against invalid name value block headers.

If a badly behaved server sends us a negative number
of name value blocks, we should throw an IOException and
not a RTE (which will be thrown when we try to construct an
ArrayList with a negative size).
Merge branch 'master' of https://github.com/narayank/okhttp
Merge pull request #127 from narayank/master

Guard against invalid name value block headers.