Fix an integer overflow by only calculating physical coordinates of
the origin of the axes when those additional axes are actually used
(i.e., when the "a" option is specified and the drawn axis is
going to be inside the viewport).
This fix solves an integer overflow issue discovered by Hazen for page
5 and larger for example 29 where the 0., 0., world coordinates of the
unused axes generated with plbox were so far from the actual X
world-coordinate range of the plot that integer overflow occurred when
calculating the unused physical coordinates corresponding the the
0.,0., world coordinates of the axes origin.