fix: Fix -Werror=nonnull-compare warning in thread affinity assertion

[Holzhaus]

Using gcc 12.2.0, passing a nonnull argument as parameter to the
DEBUG_ASSERT_QOBJECT_THREAD_AFFINITY macro leads to the following
compiler warning (or error if the build system was configured with
-DWARNINGS_FATAL=ON):

In file included from mixxx/src/preferences/configobject.h:11,
                 from mixxx/src/util/sandbox.h:10,
                 from mixxx/src/library/coverart.h:7,
                 from mixxx/src/library/coverartcache.h:9,
                 from mixxx/src/library/libraryfeature.h:15,
                 from mixxx/src/library/baseexternallibraryfeature.h:7,
                 from mixxx/src/library/baseexternallibraryfeature.cpp:1:
mixxx/src/library/trackcollection.h: In member function ‘PlaylistDAO& TrackCollection::getPlaylistDAO()’:
mixxx/src/util/assert.h:55:39: error: ‘nonnull’ argument ‘this’ compared to NULL [-Werror=nonnull-compare]
   55 | #define DEBUG_ASSERT(cond) ((!(cond)) ? mixxx_debug_assert(#cond, __FILE__, __LINE__, ASSERT_FUNCTION) : mixxx_noop())
      |                            ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mixxx/src/util/thread_affinity.h:14:5: note: in expansion of macro ‘DEBUG_ASSERT’
   14 |     DEBUG_ASSERT(pObject);                            \
      |     ^~~~~~~~~~~~
mixxx/src/library/trackcollection.h:56:9: note: in expansion of macro ‘DEBUG_ASSERT_QOBJECT_THREAD_AFFINITY’
   56 |         DEBUG_ASSERT_QOBJECT_THREAD_AFFINITY(this);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mixxx/src/library/trackcollectionmanager.h: In member function ‘TrackCollection* TrackCollectionManager::internalCollection()’:
mixxx/src/util/assert.h:55:39: error: ‘nonnull’ argument ‘this’ compared to NULL [-Werror=nonnull-compare]
   55 | #define DEBUG_ASSERT(cond) ((!(cond)) ? mixxx_debug_assert(#cond, __FILE__, __LINE__, ASSERT_FUNCTION) : mixxx_noop())
      |                            ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mixxx/src/util/thread_affinity.h:14:5: note: in expansion of macro ‘DEBUG_ASSERT’
   14 |     DEBUG_ASSERT(pObject);                            \
      |     ^~~~~~~~~~~~
mixxx/src/library/trackcollectionmanager.h:40:9: note: in expansion of macro ‘DEBUG_ASSERT_QOBJECT_THREAD_AFFINITY’
   40 |         DEBUG_ASSERT_QOBJECT_THREAD_AFFINITY(this);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
make[2]: *** [CMakeFiles/mixxx-lib.dir/build.make:2624: CMakeFiles/mixxx-lib.dir/src/library/baseexternallibraryfeature.cpp.o] Error 1

This can easily be solved by not asserting that the pointer is not
nullptr. Even if that macro is used with a regular (= not nonnull
pointer), that assetion is unnecessary because the worst that can happen
is a crash (which also makes the developer aware of a problem). When
debug assertions are disabled, the resulting instructions are the same
anyway.

[Swiftb0y]

Thanks