lavc: set the default rc_initial_buffer_occupancy

rc_buffer_size is not set before.

Solve the initial the rate control underflow issue reported in
bug 222.

CC: libav-stable@libav.org

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
(cherry picked from commit bff3607547fdbb6e32b3830a351e6a33280c1e0d)

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>