fix out-of-bound read during alpha-plane decoding
fix out-of-bound read during alpha-plane decoding

With -bypass_filter switched on, the lossless-compressed
data is decoded ahead of time (before being transformed and
display). Hence, the last row was called twice.

http://code.google.com/p/webp/issues/detail?id=193

Change-Id: I9e13f495f6bd6f75fa84c4a21911f14c402d4b10

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