basic/process-util: we need to take the shorter of two strings

basic/process-util: we need to take the shorter of two strings

==30496== Conditional jump or move depends on uninitialised value(s)
==30496==    at 0x489F654: memcmp (vg_replace_strmem.c:1091)
==30496==    by 0x49BF203: getenv_for_pid (process-util.c:678)
==30496==    by 0x4993ACB: detect_container (virt.c:442)
==30496==    by 0x182DFF: test_get_process_comm (test-process-util.c:98)
==30496==    by 0x185847: main (test-process-util.c:368)
==30496==