PanelCorner.setStyleParent() can fail with signals are missing.

PanelCorner.setStyleParent() can crash if the button doesn't have
a style-changed signal:

    JS ERROR: Exception in callback for signal: updated: Error: No signal 'style-changed' on object 'ClutterTexture'
    setStyleParent@resource:///org/gnome/shell/ui/panel.js:588:48
    wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22
    _updatePanel@resource:///org/gnome/shell/ui/panel.js:1077:13
    wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22
    _emit@resource:///org/gnome/gjs/modules/signals.js:128:27
    _sync@resource:///org/gnome/shell/ui/sessionMode.js:206:9
    wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22
    pushMode@resource:///org/gnome/shell/ui/sessionMode.js:168:9
    wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22
    activate@resource:///org/gnome/shell/ui/screenShield.js:1279:17
    wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22
    lock@resource:///org/gnome/shell/ui/screenShield.js:1328:9
    wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22
    _prepareForSleep@resource:///org/gnome/shell/ui/screenShield.js:731:17
    wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22
    _emit@resource:///org/gnome/gjs/modules/signals.js:128:27
    _prepareForSleep@resource:///org/gnome/shell/misc/loginManager.js:180:9
    wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22
    _emit@resource:///org/gnome/gjs/modules/signals.js:128:27
    _convertToNativeSignal@resource:///org/gnome/gjs/modules/overrides/Gio.js:117:5

Fail gracefully when that happens.
4 jobs for !539 with misc-fixes in 20 seconds (queued for 3 seconds)
latest detached
Status Job ID Name Coverage
  Review
failed #295835
check_commit_log

00:00:20

 
  Build
skipped #295836
build
 
  Test
skipped #295837
test
skipped #295838
test-pot
 
Name Stage Failure
failed
check_commit_log Review
 * [new tag]         3.32.1     -> 3.32.1
Checking out 9ea298f4 as misc-fixes...

Skipping Git submodules setup
$ ./.gitlab-ci/check-commit-log.sh
From https://gitlab.gnome.org/GNOME/gnome-shell
* branch master -> FETCH_HEAD
Missing merge request or issue URL on commit 9ea298f4
ERROR: Job failed: exit code 1