adpcm: Avoid reading out of bounds in the IMA QT trellis encoder

This was broken in 095be4fb - samples+ch (for the previous
non-planar case) equals &samples_p[ch][0]. The confusion
probably stemmed from the IMA WAV case where it originally
was &samples[avctx->channels + ch], which was correctly
changed into &samples_p[ch][1].

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 3d79d0c93e5b37a35b1b22d6c18699c233aad1ba)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
adpcm: Write the proper predictor in trellis mode in IMA QT

The actual predictor value, set by the trellis code, never
was written back into the variable that was written into
the block header. This was accidentally removed in b304244b.

This significantly improves the audio quality of the trellis
case, which was plain broken since b304244b.

Encoding IMA QT with trellis still actually gives a slightly
worse quality than without trellis, since the trellis encoder
doesn't use the exact same way of rounding as in
adpcm_ima_qt_compress_sample and adpcm_ima_qt_expand_nibble.

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 0776e0ef6ba4160281ef3fabea43e670f3792b4a)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>