cli/show: avoid empty write to stdout in format_part_raw
Previously if the input was exactly a multiple of the internal buffer
size, notmuch would attempt to fwrite nothing to stdout, but still
expected fwrite to return 1, causing a failure that looked like this:
$ notmuch show --format=raw id:87o96f1cya.fsf@codeaurora.org
...entire message shown as expected..
Error: Write failed
$ echo $?
1
To fix the problem don't call fwrite at all when there's nothing to
write.
Amended by db: add some tests of message sizes likely to cause this
problem.