Detect invalid highbd iht input

Do nothing in vp9_highbd_iht#x#_##_add_c when input magnitude is beyond
20 bits. Note that, sign bit is not included here.

In the 20 bits, we use 12 bits for input signal, 7 bits for forward
transform amplification, and 1 bit for contingency in rounding and
quantizing

BUG=https://bugs.chromium.org/p/webm/issues/detail?id=1286

Change-Id: I332c6f68df4614fc2e7d2dc4c5bb0d0cff8a245c
1 file changed