lavfi/mp/mcdeint: avoid uninited data read

Do not read padding or out-of-buffer values when computing the output
value for a pixel close to the image buffer edge.

This avoids non visible artifacts which affected the output checksum.

See thread:
Subject: [FFmpeg-devel] [PATCH] lavfi/mp/mcdeint: avoid uninited data read
Date: Thu, 30 May 2013 18:57:14 +0200
