FIXED: Possible crash in arg(-,+,?).
FIXED: Possible crash in arg(-,+,?).

Bug introduced in 32bbcdac54c8bac721eeced8bd8c5d2d2a933d6d

mndrix/swipl-devel,mndrix/swipl-devel,koryonik/swipl-devel,mndrix/swipl-devel,jn7163/swipl-devel,jn7163/swipl-devel,jn7163/swipl-devel,koryonik/swipl-devel,edechter/swipl-devel,mndrix/swipl-devel,edechter/swipl-devel,jn7163/swipl-devel,mndrix/swipl-devel,edechter/swipl-devel,koryonik/swipl-devel,edechter/swipl-devel,edechter/swipl-devel,koryonik/swipl-devel,jn7163/swipl-devel,koryonik/swipl-devel