MacOS X port: do not free argv if it was not allocated.

Darwin revealed a bug in our free_proc_title() function, which was
added to procitle.c to work well with valgrind: we free argv pointer
even if it wasn't allocated (none of PS_USE_CLOBBER_ARGV/PS_USE_CHANGE_ARGV
was set).

Don't free memory if it wasn't allocated.

Another issue is understanding why Darwin doesn't define
PS_USE_SETPROCTITLE and why our FreeBSD port didn't hit this bug.