fix the dichotomy loop
fix the dichotomy loop

we were reading past the end of the dqs[] array.

reported by Mathias Schindler (on cygwin only)
http://code.google.com/p/webp/issues/detail?id=71

Change-Id: Ib38c4c139e3cac3e8915626d63e16b403d6bbd63

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