gstquery: handle failure to get ID value from structure

gst_structure_id_get_value() will return NULL if it can't find the
given field. This will cause g_value_get_string() to dereference
a NULL pointer if glib was compiled with G_DISABLE_CHECKS.

FALSE should also be returned in this case to indicate a parsing
failure and handled as appropriate by callers.
36 jobs for !751 with fix-query-parse-context-type in 56 seconds (queued for 3 seconds)
latest detached
Status Job ID Name Coverage
  Build Docker
passed #7155569
alpine amd64 manifest builder docker

00:00:33

passed #7155568
android docker

00:00:32

passed #7155567
cerbero fedora amd64 docker

00:00:19

passed #7155566
fedora amd64 docker

00:00:20

passed #7155570
gst-indent amd64 docker

00:00:32

 
  Preparation
passed #7155573
check allow-edit

00:00:13

passed #7155572
gst indent

00:00:22

manual #7155571
manual
manifest
 
  Build
created #7155582
gstreamer
build cerbero cross-android universal
created #7155586
gst-ios-14.3
build cerbero cross-ios universal
created #7155583
gstreamer
build cerbero cross win32
created #7155584
gstreamer
build cerbero cross win64
created #7155581
gstreamer
build cerbero fedora x86_64
created #7155585
gst-macos-11.1
build cerbero macos x86_64
created #7155587
docker windows gstreamer-windows 1809
build cerbero msvc x86_64
created #7155577
build clang fedora x86_64
created #7155580
docker windows 1809 allowed to fail manual
build msys2
created #7155574
build nodebug fedora x86_64
created #7155575
build static fedora x86_64
created #7155576
build static nodebug fedora x86_64
created #7155578
docker windows 1809
build vs2017 amd64
created #7155579
docker windows 1809
build vs2017 x86
 
  Test
created #7155588
gstreamer
check fedora
created #7155589
gstreamer
integration testsuites fedora 1/4
created #7155590
gstreamer
integration testsuites fedora 2/4
created #7155591
gstreamer
integration testsuites fedora 3/4
created #7155592
gstreamer
integration testsuites fedora 4/4
created #7155597
gstreamer
valgrind bad
created #7155594
gstreamer
valgrind base
created #7155593
gstreamer
valgrind core
created #7155598
gstreamer
valgrind ges
created #7155595
gstreamer
valgrind good
created #7155596
gstreamer
valgrind ugly
 
  Integrate
created #7155599
cross-android universal examples
created #7155601
gst-ios-14.3
cross-ios universal examples
created #7155600
documentation