Where clang static analysis or gcc -Wmaybe-uninitialized warns of
uninitialized values, assign 0 to ints, MB_MODE_COUNT to
MB_PREDICTION_MODE, and B_MODE_COUNT to B_PREDICTION_MODE.

Assert that the modes have been changed from the invalid value by
the end of the function.

Change-Id: Ib11e1ffb08f0a6fe4b6c6729dc93b83b1c4b6350