GWin32RegistryKey: Move assertions

While these assertions look right at the first glance,
they actually crash the program. That's because GObject
insists on initializing all construct-only properties
to their default values, which results in
g_win32_registry_key_set_property() being called multiple
times with NULL string, once for each unset property.

If "path" is actually set by the caller, a subsequent
call to set "path-utf16" to NULL will fail an assertion,
since absolute_path is already non-NULL.

With assertions moved the set-to-NULL calls bail out before
an assertion is made.
16 jobs for win32reg-assertions in 44 minutes and 3 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Style Check
passed #753980
check-todos

00:01:01

passed #753979
style-check-diff

00:00:59

 
  Build
passed #753985
cross-android_api21_arm64

00:02:19

passed #753986
cross-android_api28_arm64

00:02:20

passed #753987
cross-mingw64

00:02:00

passed #754009
debian-stable-x86_64

00:08:58

passed #754010
fedora-x86_64

00:19:39

passed #753984
G_DISABLE_ASSERT

00:05:56

passed #753983
installed-tests

00:04:05

passed #753988
win32-ps
msys2-mingw32

00:08:30

passed #753989
win32-ps
vs2017-x64

00:08:26

failed #753982
debian-stable-x86_64

00:05:51

failed #753981
fedora-x86_64

00:16:59

 
  Coverage
passed #753990
coverage

00:01:35

78.2%
 
  Analysis
passed #753992
scan-build

00:07:26

failed #753991
allowed to fail
valgrind

00:13:15

 
Name Stage Failure
failed
valgrind Analysis
Uploading artifacts...
_build/config.h: found 1 matching files

_build/glib/glibconfig.h: found 1 matching files

_build/meson-logs: found 7 matching files

Uploading artifacts to coordinator... ok
id=753991 responseStatus=201 Created token=5T9eot5L
Uploading artifacts...
_build/valgrind-report.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=753991 responseStatus=201 Created token=5T9eot5L
ERROR: Job failed: exit code 1