analysis: handle no-valid-merge more cleanly, simplify compressInterCU_rd0_4

checkMerge2Nx2N_rd0_4() will set md.bestMode if any of them were valid, else it
will be left NULL. Fixes some rare decoder asserts when no valid modes is coded
frameencoder: handle non-present CUs correctly when collecting stats
analysis: add emergency intra check for --pmode

if all other choices somehow fail, we should always be able to code intra