Fix CID 1435996: use the proper % type to render the size.
Also use opal_output(), not fprintf(). For debug builds, abort
without dumping core (dumping core is very unfriendly when running
thousands of automated tests) -- the stderr output is sufficient to
find the coding error. For non-debug builds, truncate the key and
emit a warning that it almost certainly will not work properly.
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>