avoid crash with option -0

avoid crash with option -0

Instead of using the possible option character '0' (ASCII value 0x30)
to indicate a long option without a short alias, use the value 0 as
indicator.  This value is treated specially by the used option parsing
code, and might even be the intended value to use for long-only options.

Before, using option `-0` sometimes, but not always, resulted in a
crash with `Segmentation fault (core dumped)`.

This fixes GitHub issue #456.