Fix parsing of senc box

[paroga]

IMHO the duplicated PrefixedArray isn't a clean solution, but I wasn't able to fix it otherwise.

[jairoxyz]

Thanks, very helpful fix, which I hoped would solve parsing senc boxes in serveral test files I have but even with your fix it won't find any senc boxes. For example in this file https://github.com/Eyevinn/mp4ff/raw/master/mp4/testdata/prog_8s_enc_dashinit.mp4, which I tested with an online parser and it definitetly has senc boxes under the moof->traf boxes. Any chance you could try to parse it?