mxf2raw: check value of frames parameter for rdd6 option

[Matthew-Marks]

Minor - please feel free to wontfix. You can put any old rubbish in the frames mxf2raw commandline parameter and as long as there's a number in there it seems to be happy. Specifically, if you put in a range separated by a comma rather than a hyphen, it doesn't complain but it doesn't do what you might expect either. Should only allow digits and optional hyphen.