ppp: Fix seg fault when using pppol2tp

PPPD crashes (SEGV) when the dump or dryrun options are specified and an option
is internally defined as "o_special" with an option flag of "OPT_A2STRVAL".
As the option value is not saved when the parameter is processed, a reference
to the option will result into a crash (e.g. when printing).

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

SVN-Revision: 45263