Fix 12 TAP convolution bug

Priviously, we do 12-tap interpolation even there is no sub pixel,
This could cause a bug becuase decoder doesn't extend border when there
is no sub pixel. In this situation, if we still do interpolation, we
will access the border extension which doesn't exist and cause a
memory error

Change-Id: I55b879722f0a10c5d13261bd9617a75c826a2418