Fix warnings about invalid reads in AVX2 ipol
Fix warnings about invalid reads in AVX2 ipol

AVX2 filter functions read pixels in chunks of 8 or 16 bytes. At the end
of the block, the read goes out of the bounds of the pixels array. The
extra pixels do not affect the result.

Fixes valgrind complaining about the invalid reads by allocating 5 extra
pixels in kvz_get_extended_block_avx2

ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar