AddressSanitizer: alloc-dealloc-mismatch on Clang 12/13

Clang 12 and 13 report a alloc-dealloc-mismatch on CI; no code changes though.

SUMMARY: AddressSanitizer: alloc-dealloc-mismatch (/builds/offa/plug/build/test/MustangTest+0x4aea12) 
==541==HINT: if you don't care about these errors you may set ASAN_OPTIONS=alloc_dealloc_mismatch=0
==541==ABORTING
make[3]: *** [test/CMakeFiles/unittest.dir/build.make:72: test/CMakeFiles/unittest] Error 1
make[2]: *** [CMakeFiles/Makefile2:539: test/CMakeFiles/unittest.dir/all] Error 2
make[1]: *** [CMakeFiles/Makefile2:546: test/CMakeFiles/unittest.dir/rule] Error 2
make: *** [Makefile:348: unittest] Error 2

Error appears on GH and GL CI.