avutil/opt: dont crash on av_opt_set_dict() with NULL

This allows calling the function without the need to check if the
dictionary contains any entries

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/avio: also set generic URL context options

This should have no effect currently as there are no such options yet.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>