Fix FILL with PLOTHIST when axis is inverted

Fix use of FILL in PLOTHIST with an axis is inverted (i.e. xcrange[0] >
xcrange[1]).  Add check for inverted axis, and correctly force XHIST values to
be within the plot range.

This fix is useful for Galactic Longitude plots, among other things.

	modified:   pro/plot/plothist.pro