When writing the compressed header, prob_diff_update() was called
for compound_type_prob[] for every defined block size, even though
luma never uses block sizes smaller than 4x4.

This fixes is_any_masked_compound_used() and
is_interinter_compound_used() to properly return 0 for chroma-only
block sizes, and then uses these functions to guard the probability
updates in write_compressed_header() and read_compressed_header(),
the same way the actual compound type values are guarded in
read_inter_block_mode_info() and pack_inter_mode_mvs().

Change-Id: Ib521cf53f9ec166ef634609c8b47c5814b6a9ff5