Coverity complained about the lack of a check on the return value in
parse_jsonb_index_flags' last call of JsonbIteratorNext.  Seems like
a reasonable gripe to me, especially since the code is depending on
that being WJB_DONE to not leak memory, so add a check.

In passing, improve a couple other places where the result was being
ignored, either by adding an assert or at least a cast to void.

Also, don't spell "WJB_DONE" as "0".  That's horrid coding style,
and it wasn't consistent either.