demux: mp4: add lpcm stsd
demux: mp4: do stsd v2 checks
demux: mp4: fix stsd v2 reading
demux: mp4: go forward on null sample

Was regression with that tricky one:
http://streams.videolan.org/issues/10981/6.mp4
demux: mp4: name field
demux: mp4: don't duplicate vars
demux: mp4: unify and simplify samples reading
demux: mp4: remove debug string