isosurf: fix debug print of which slice had mem error.
jsdd: remove debug print
silo: clean up json file's memory.
silo: implement buffer so we don't call with short reads.
posix: ensure mode is initialized when RO.

shouldn't be used when read only, but just in case, let's give it
a defined value.
our custom json: assert we have dimensions.

mostly to get around a clang analyzer problem.