Fix decode bug for rgbA_4444/RGBA_4444 color-modes.

The WEBP_SWAP_16BIT_CSP flag needs to be honored while filling the Alpha (4 bits)
data in the destination buffer and while pre-multiplying the alpha to RGB colors.

Change-Id: I3b07307d60963db8d09c3b078888a839cefb35ba
WEBP_SWAP_16BIT_CSP: remove code dup

and prepare for potentially supporting both RGBA4444 and BARG4444

Change-Id: If5200289bc6338757a2ceb2df1a19de732595052
Don't dereference NULL, ensure HashChain fully initialized

Found by clang's static analyzer, they look validly uninitialized
to me.

Change-Id: I650250f516cdf6081b35cdfe92288c20a3036ac8
Merge "Don't dereference NULL, ensure HashChain fully initialized"