Fix for negative `toItem` variable

[tchudyk]

Sometimes when I'm using lastest RichTextFx, when I select all text in editor, and cut it (ctrl+x), there is throw exception because of negative toItem variable.

This solution probably is not perfect, but it solve this problem.

I see also here "validation" to not pass values bigger than cells.size(), so adding validations to not pass values lower then 0 make also sense.

Here is a stacktrace for this exception:

Exception in thread "JavaFX Application Thread" java.lang.IndexOutOfBoundsException: [-2147483648, 1) is not a valid range within [0, 1)
    at org.reactfx.util.Lists.checkRange(Lists.java:128)
    at org.reactfx.util.Lists.checkRange(Lists.java:123)
    at org.reactfx.collection.MemoizationListImpl.forget(MemoizationList.java:184)
    at org.fxmisc.flowless.CellListManager.cropTo(CellListManager.java:86)
    at org.fxmisc.flowless.CellPositioner.cropTo(CellPositioner.java:29)
    at org.fxmisc.flowless.Navigator.cropToNeighborhoodOf(Navigator.java:254)
    at org.fxmisc.flowless.Navigator.placeStartAtMayCrop(Navigator.java:227)
    at org.fxmisc.flowless.Navigator.visit(Navigator.java:182)
    at org.fxmisc.flowless.MinDistanceTo.accept(TargetPosition.java:198)
    at org.fxmisc.flowless.Navigator.layoutChildren(Navigator.java:80)
    at javafx.graphics/javafx.scene.Parent.layout(Parent.java:1207)
    at org.fxmisc.flowless.VirtualFlow.layoutChildren(VirtualFlow.java:257)
    at javafx.graphics/javafx.scene.Parent.layout(Parent.java:1207)
    at javafx.graphics/javafx.scene.Parent.layout(Parent.java:1214)
    at javafx.graphics/javafx.scene.Parent.layout(Parent.java:1214)
    at javafx.graphics/javafx.scene.Parent.layout(Parent.java:1214)
    at javafx.graphics/javafx.scene.Parent.layout(Parent.java:1214)
    at javafx.graphics/javafx.scene.Scene.doLayoutPass(Scene.java:576)
    at javafx.graphics/javafx.scene.Scene$ScenePulseListener.pulse(Scene.java:2476)
    at javafx.graphics/com.sun.javafx.tk.Toolkit.lambda$runPulse$2(Toolkit.java:413)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at javafx.graphics/com.sun.javafx.tk.Toolkit.runPulse(Toolkit.java:412)
    at javafx.graphics/com.sun.javafx.tk.Toolkit.firePulse(Toolkit.java:439)
    at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:563)
    at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:543)
    at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.pulseFromQueue(QuantumToolkit.java:536)
    at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.lambda$runToolkit$11(QuantumToolkit.java:342)
    at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run$$$capture(InvokeLaterDispatcher.java:96)
    at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java)
    at javafx.graphics/com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
    at javafx.graphics/com.sun.glass.ui.gtk.GtkApplication.lambda$runLoop$11(GtkApplication.java:277)
    at java.base/java.lang.Thread.run(Thread.java:831)

[Jugen]

@tchudyk Thank you very much :-)