aacdec: set AVFrame sample_rate

AVFrame.sample_rate is set in ff_get_buffer, but aacdec calls
ff_get_buffer before the samplerate is known. So it needs to be
set again before returning the frame.
asfdec: check ff_get_guid() return values during seeking

Hitting EOF during seeking is quite likely. Fixes use of uninitialized
data during fate-seek-lavf-asf.
asfdec: short-circuit seeking to the start of stream

Bug-id: 43

Signed-off-by: Janne Grunau <janne-libav@jannau.net>
asfdec: fix seeking with fragmented packets

After seeking fragments with an offset > 0 must be skipped to correctly
assemble packets.

Bug-Id: 43