Don't bother computing check value after successful inflateSync().

Don't bother computing check value after successful inflateSync().

inflateSync() is used to skip invalid deflate data, which means
that the check value that was being computed is no longer useful.
This commit turns off the check value computation, and furthermore
allows a successful return if the compressed data terminated in a
graceful manner. This commit also fixes a bug in the case that
inflateSync() is used before a header is ever processed. In that
case, there is no knowledge of a trailer, so the remainder is
treated as raw.