vf_hqdn3d: fix uninitialized variable use

CC:libav-stable@libav.org
(cherry picked from commit d0a863ac891eae49ceaa4de7f759270bc87e668d)

Conflicts:

	libavfilter/vf_hqdn3d.c
vf_gradfun: fix uninitialized variable use

CC:libav-stable@libav.org
(cherry picked from commit 887d31d455915b6bde6814063384dafdee61164c)

Conflicts:

	libavfilter/vf_gradfun.c
hqdn3d: Fix out of array read in LOWPASS

CC:libav-stable@libav.org
Signed-off-by: Anton Khirnov <anton@khirnov.net>
(cherry picked from commit 5b3c1aecb253828d09fa9825c5a4aed97badf086)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>