Fix: unplug memory leak that causes popt-0.13 to segfault

popt-0.16 poptGetArg() causes a memory leak, but freeing that memory
goes against popt(3) documented use, and against the 0.13 behavior,
which triggers a segfault on free(). So leave this small,
one-time-per-execution, memory leak in place.

Fixes #380

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>