mov: Trim dref absolute path

Samples produced by Omneon (Harmonic) store external references with
paths ending with 0s. Such movs cannot be loaded properly since every
0 is converted to '/', to keep the same parsing code for dref type 2
and type 18: this makes the external reference point to a non-existing
direactory, rather than to the actual referenced file.

Add a brief trimming loop that drops all ending 0s before trying to
parse the external reference path.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>