!selection.show => return null from getSelection

If we've called `clearSelection`, then calling `getSelection` shouldn't
return the old selection values. When calling `getSelection` before a
selection is made, `null` is returned. It seemed appropriate to do the
same if `selection.show` was falsy.
Signed-off-by: Nick Campbell <nicholas.j.campbell@gmail.com>

jquery.flot.selection.js

@@ -147,6 +147,8 @@ The plugin allso adds the following methods to the plot object:
             if (!selectionIsSane())
                 return null;
             
+            if (!selection.show) return null;
+
             var r = {}, c1 = selection.first, c2 = selection.second;
             $.each(plot.getAxes(), function (name, axis) {
                 if (axis.used) {