ffmpeg: fix CVE-2020-22033 and CVE-2020-22019

ffmpeg: fix CVE-2020-22033 and CVE-2020-22019

avfilter/vf_vmafmotion: Check dimensions

Fixes: out of array access
Fixes: Ticket8241
Fixes: Ticket8246
Fixes: CVE-2020-22019
Fixes: CVE-2020-22033
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
CVE: CVE-2020-22033
CVE: CVE-2020-22019
Upstream-Status: Backport [82ad1b76751bcfad5005440db48c46a4de5d6f02]

Signed-off-by: Tony Tascioglu <tony.tascioglu@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>