Improvement: clang-tidy fixes #710

Job	Run time
fine_grained_trigger	9s
t8code_doxygen_check / check_documentation	9s
t8code_tarball / build_tarball	10s
t8code_tarball / test_tarball	1s
preparation	-1s
t8code_w_clang_and_ompi_tests	-1s
t8code_tests	0s
t8code_linkage_tests	0s
t8code_api_tests	0s
t8code_w_shipped_submodules_tests	-1s
sc_p4est_tests	0s
t8code_code_coverage	0s
t8code_valgrind_tests	0s
	26s

Provide feedback