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>

libavformat/mxfdec.c

@@ -792,7 +792,8 @@ static void mxf_read_pixel_layout(AVIOContext *pb, MXFDescriptor *descriptor)
         if (ofs <= 14) {
             layout[ofs++] = code;
             layout[ofs++] = value;
-        }
+        } else
+            break;  /* don't read byte by byte on sneaky files filled with lots of non-zeroes */
     } while (code != 0); /* SMPTE 377M E.2.46 */
 
     ff_mxf_decode_pixel_layout(layout, &descriptor->pix_fmt);