fix uninitialized read in encode breakout

Change I3430820b performed an uninitialized read when
encode_breakout == 0, since the sum and sse wouldn't be set:

   if(x->encode_breakout)
       VARIANCE_INVOKE(..., get16x16var)(..., &sum, &sse);
   if (cpi->active_map_enabled && x->active_ptr[0] == 0) {
       ...
   } else if (sse < x->encode_breakout)

Change-Id: I915eb76d1227b4b6d1137a0dedf2c143860098a2