Valgrind was complaining:

==8428== 32,816 bytes in 1 blocks are definitely lost in loss record 983 of 985
==8428==    at 0x4C2EDA7: malloc (vg_replace_malloc.c:299)
==8428==    by 0x96779A5: __alloc_dir (in /usr/lib64/libc-2.27.so)
==8428==    by 0x9677AAC: opendir_tail (in /usr/lib64/libc-2.27.so)
==8428==    by 0x59EC50: bpftrace::list_dir(...)
==8428==    by 0x59ED5F: bpftrace::list_probes(...)
==8428==    by 0x594993: main