swscale: Use alpha from the right row in yuva2rgba_c

Every other pixel had the alpha channel taken from the wrong
row.

This fixes bug 504.

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 6e293d111fcad27d52a2ef5ad77b1009f1743396)

Signed-off-by: Martin Storsjö <martin@martin.st>