lavc/options: fix rc_eq leak

rc_eq is an option, so it is copied by av_opt_copy(dest, src); above.
lavu/dict: add av_dict_get_string

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
avformat/avidec: fix handling dv in avi

Fixes Ticket4086

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