opt: re-enable memleak fix for duplicated options.

The memleak fix in a1bcc76e was causing problems when the original
string wasn't NULL or av_malloc'ed, thus was disabled. It seems the only
OPT_STRING occurence where it needed to be fixed is in ffserver. This
commit fixes that specific problem and so re-enable the original code.
CrystalHD: Port to ref-counted frame API.

Signed-off-by: Philip Langdale <philipl@overt.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>