smartctl.cpp, smartd.cpp: Fix segfault on missing option argument (#1830).

getopt*() from musl libc sets 'optind = argc + 1' if the argument of
a short option is missing.

git-svn-id: https://svn.code.sf.net/p/smartmontools/code/trunk@5613 4ea69e1a-61f1-4043-bf83-b5c94c648137