Fixes #774 (#775)

* Fix replace selected text with longer pasted text bug #774

Author: Aleksandr Beschasnyi

richtextfx/src/main/java/org/fxmisc/richtext/SelectionImpl.java

@@ -260,6 +260,9 @@ public SelectionImpl(String name, GenericStyledArea<PS, SEG, S> area, int startP
                                     ? indexOfChange
                                     : finalEnd + netLength;
                         }
+                        if (finalStart > finalEnd) {
+                            finalStart = finalEnd;
+                        }
                     } else {
                         // force-update internalSelection in case empty selection is
                         // at the end of area and a character was deleted