avformat/rtsp: add av_assert0() to clarify that the else case is supposed to be unreachable

If its reachable then theres a bug as err would be uninitialized

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/rtsp: pass return code from ffurl_open() on its failure

Previously, AVERROR(EIO) was returned. Now the value is passed from
lower level, thus it is possible to distinguish ECONNREFUSED, ETIMEDOUT,
ENETUNREACH etc.

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