libdockapp: removed assertion that short-form options have length 2.
The mixed-option parsing code includes an assertion that short-form
options have a length of two. However, there is no other validation of
this requirement and some dock-apps do not comply with it, which means
that if one exec's them with an unrecognized option or a mix of short
options, the assertion fails and the app aborts.
For example:
$ /usr/bin/wmail --help | grep '\<display\>'
-display <string> display to use
$ /usr/bin/wmail --blah
wmail: daargs.c:126: contains: Assertion `strlen(needle) == 2' failed.
Aborted
Since there is no explicit statement of this requirement, let's replace
the assertion with a conditional.