Fix multiple crashes on Linux

[droidmonkey]

Fix crash when multiple dbus unlock calls are issued

• Fixes Segmentation fault when attempting to unlock the database via dbus on ScreenSaver signal #11512

Fix crash on Linux when database is closed without hardware key present

• Fixes Linux: Crash when two databases use hardware key and one is closed without key present #11450

Testing strategy

Tested before/after change, crash no longer occurs with guard in place

Type of change

• ✅ Bug fix (non-breaking change that fixes an issue)

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 63.88%. Comparing base (bf856d2) to head (ecfbead).
Report is 9 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/gui/DatabaseOpenWidget.cpp	33.33%	2 Missing ⚠️
src/gui/osutils/DeviceListener.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop   #11513   +/-   ##
========================================
  Coverage    63.88%   63.88%           
========================================
  Files          362      362           
  Lines        38140    38143    +3     
========================================
+ Hits         24363    24365    +2     
- Misses       13777    13778    +1     

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