checkasm: cdef: Don't use uninitialized buffer contents for the test

This makes sure the test actually is deterministic based on the
seed value.
16 jobs for master in 8 minutes and 35 seconds (queued for 25 seconds)
Status Job ID Name Coverage
  Style
passed #250818
amd64 debian
style-check

00:00:21

 
  Build
passed #250819
amd64 debian
build-debian

00:00:35

passed #250826
debian aarch64
build-debian-aarch64

00:01:24

passed #250827
debian aarch64
build-debian-aarch64-clang-5

00:01:06

passed #250820
amd64 debian
build-debian-static

00:00:36

passed #250829
debian aarch64
build-debian-werror

00:00:32

passed #250821
amd64 debian
build-debian32

00:00:30

passed #250828
macos
build-macos

00:00:27

passed #250824
win32
build-win-arm32

00:00:26

passed #250825
win64
build-win-arm64

00:00:30

passed #250822
win32
build-win32

00:00:33

passed #250823
win64
build-win64

00:00:37

 
  Test
passed #250830
amd64 debian
test-debian

00:00:45

passed #250831
amd64 debian
test-debian-asan

00:02:10

passed #250832
amd64 debian
test-debian-msan

00:01:00

passed #250833
amd64 debian
test-debian-ubsan

00:01:19