mxfdec: Fix a potential DoS vector in mxf_read_pixel_layout()

There's a a potential DoS problem in this function. Say an MXF file is
created with a PixelLayout with a long run of non-zeroes. Such a file could be
sent quickly (packed) over the net and would unpack quite fast. mxfdec would
then read it byte-by-byte, which would take considerable time.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
