bej_decoder: Add a limit on number of operations

This is to prevent the decoder from taking on a huge number of
operations from a single input, that could cause the main while loop for
bejDecode to run for a long time.

Tested: Unit test passes

Change-Id: Idb68b706d3634d1874d1814f17420076672d21ee
Signed-off-by: Brandon Kim <brandonkim@google.com>