Fix crash in SSSE3 inverse transform

The 32x32 identity_identity transform would corrupt the stack, including
the return address, when compiling with a 16-byte stack alignment on
non-Windows systems.
21 jobs for master in 5 minutes and 56 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Style
passed #273706
amd64 debian
style-check

00:00:20

 
  Build
passed #273707
amd64 debian
build-debian

00:00:40

passed #273715
debian aarch64
build-debian-aarch64

00:01:34

passed #273716
debian aarch64
build-debian-aarch64-clang-5

00:01:09

passed #273719
debian armv7
build-debian-armv7

00:01:37

passed #273720
debian armv7
build-debian-armv7-clang-5

00:01:24

passed #273708
amd64 debian
build-debian-static

00:00:39

passed #273718
debian aarch64
build-debian-werror

00:00:35

passed #273709
amd64 debian
build-debian32

00:00:36

passed #273717
macos
build-macos

00:00:37

passed #273721
amd64 debian allowed to fail
build-ubuntu-snap

00:00:59

passed #273713
amd64 debian
build-win-arm32

00:00:27

passed #273714
amd64 debian
build-win-arm64

00:00:29

passed #273710
amd64 debian
build-win32

00:00:40

passed #273711
amd64 debian
build-win32-unaligned-stack

00:00:39

passed #273712
amd64 debian
build-win64

00:00:47

 
  Test
passed #273722
amd64 debian
test-debian

00:00:55

passed #273723
amd64 debian
test-debian-asan

00:02:10

passed #273724
amd64 debian
test-debian-msan

00:01:07

passed #273725
amd64 debian
test-debian-ubsan

00:01:25

passed #273726
amd64 debian
test-win64

00:00:50