Prevent filling transform size cache with uninitialized values.
The 32x32 value in case of splitmv was uninitialized. this leads to all kind of erratic behaviour down the line. Also fill in dummy values for superblocks in keyframes (the values are currently unused, but we run into integer overflows anyway, which makes detecting bad cases harder). Lastly, in case we did not find any RD value at all, don't set tx_diff to INT_MIN, but instead set it to zero (since if we couldn't find a mode, it's unlikely that any particular transform would have made that worse or better; rather, it's likely equally bad for all tx_sizes). Change-Id: If236fd3aa2037e5b398d03f3b1978fbbc5ce740e