avcodec/vp9dsp: fix overwrite by 1 in vert_left pred.

The memset following the memcpy was silencing the problem since
re-writing that same byte.

Fixes CID1108597, CID1108598, (16x16)
      CID1108599, CID1108600, (8x8)
      CID1108601, CID1108602  (32x32)

Signed-off-by: Clément Bœsch <u@pkh.me>
avformat/vobsub: fix invalid sub queue access while seeking.

If there is only 1 stream and seek isn't called with a specific stream
index, we pick the first (and only) one.

Regression since dbfe6110.

Fixes CID1108591.
fate: add synthetic swresample resampling tests

this is based on libavresample.mak

These tests do not depend on external samples

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