gparam: fix memory leak in g_param_value_defaults()

We cannot  just call

    G_PARAM_SPEC_GET_CLASS (pspec)->value_set_default (pspec, &dflt_value);

without initializing the GValue first. It would call
param_string_set_default(), which would set the pointer value
to a cloned string (which later never gets released, because
the GValue is not known to hold a string).

Fixes: 6ad799ac
11 jobs for th/fix-g-param-value-defaults-leak in 36 minutes and 11 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #494419
cross-android_api21_arm64

00:01:26

passed #494420
cross-android_api28_arm64

00:02:48

passed #494421
cross-mingw64

00:01:49

passed #494417
debian-stable-x86_64

00:05:22

passed #494416
fedora-x86_64

00:07:31

passed #494418
G_DISABLE_ASSERT

00:01:53

passed #494422
win32
msys2-mingw32

00:09:18

passed #494423
win32
vs2017-x64

00:09:31

 
  Coverage
passed #494424
coverage

00:01:38

78.0%
 
  Analysis
passed #494426
scan-build

00:17:54

failed #494425
allowed to fail
valgrind

00:24:58

 
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=494425 responseStatus=201 Created token=n_AJqhxj
Uploading artifacts...
_build/valgrind-report.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=494425 responseStatus=201 Created token=n_AJqhxj
ERROR: Job failed: exit code 1