Jakob Petsovits accepted merge request !440: applets/brightness: More robustness for surviving plugin destruction at Plasma / PowerDevil

Follow-up tweaks (c.f. !438) to further improve crash resistance when thebrightness applet's plugin is torn down while updating its displays:

• The scope guard, executed at the end of a display update loop,will now also check for aliveness to avoid writing to a member.Thanks to Anthony Fieroni for catching this.
• The aliveness check for queryAndInsertDisplay() will now beperformed after each call in the loop, instead of once at theouter while loop. More safety also for multi-monitor setups.

I'm pretty confident that this puts us at 100% coverage ofaliveness checks after co_await.

CC @anthonyfieroni, @fusionfuture (thanks for your previous reviews and thanks in advance for this follow-up!)