join.c: clean up verify_swm_set()

join.c: clean up verify_swm_set()

UBSAN complained when the parts_to_swms array had 0 length.  Clean this
up by sorting the parts first, making the verification simpler.  Also
don't bother checking compression_type and chunk_size anymore; checking
guid should be sufficient, and it doesn't really matter if the
compression formats are different since now everything will be written
out correctly anyway.