Fix Fl_Tree::insert() with pos out ouf range (#18)

The given position to insert a new item was not checked  against the
valid range so the program  could crash if a position less than zero
or greater than children() was given. The position is now clamped to
the valid range, i.e. the item is either prepended or appended.

Fixes issue #18.

src/Fl_Tree.cxx

@@ -874,6 +874,14 @@ Fl_Tree_Item* Fl_Tree::insert_above(Fl_Tree_Item *above, const char *name) {
 
 /**
  Insert a new item \p 'name' into \p 'item's children at position \p 'pos'.
+
+ If \p pos is out of range the new item is
+  - prepended if \p pos \< 0 or
+  - appended  if \p pos \> item->children().
+
+ Note: \p pos == children() is not considered out of range: the item is
+ appended to the child list.
+
  Example:
  \par
  \code

src/Fl_Tree_Item.cxx

@@ -387,10 +387,17 @@ Fl_Tree_Item *Fl_Tree_Item::add(const Fl_Tree_Prefs &prefs,
 	 : 0;				      // failed? error
 }
 
-/// Insert a new item named \p 'new_label' into current item's
-/// children at a specified position \p 'pos'.
-/// \returns the new item inserted.
-///
+/**
+  Insert a new item named \p 'new_label' into current item's
+  children at a specified position \p 'pos'.
+
+  If \p pos is out of range the new item is
+   - prepended if \p pos \< 0 or
+   - appended  if \p pos \> item->children().
+
+  \returns the new item inserted
+  \see Fl_Tree::insert()
+*/
 Fl_Tree_Item *Fl_Tree_Item::insert(const Fl_Tree_Prefs &prefs, const char *new_label, int pos) {
   Fl_Tree_Item *item = new Fl_Tree_Item(_tree);
   item->label(new_label);

src/Fl_Tree_Item_Array.cxx

@@ -109,9 +109,14 @@ void Fl_Tree_Item_Array::enlarge(int count) {
 /// Insert an item at index position \p pos.
 ///
 ///     Handles enlarging array if needed, total increased by 1.
-///     If \p pos == total(), an empty item is appended to the array.
+///     If \p pos \>= total(), the item is appended to the array.
+///     If \p pos \< 0, the item is prepended (works like pos == 0).
 ///
 void Fl_Tree_Item_Array::insert(int pos, Fl_Tree_Item *new_item) {
+  if (pos < 0)
+    pos = 0;
+  else if (pos > _total)
+    pos = _total;
   enlarge(1);
   // printf("*** POS=%d TOTAL-1=%d NITEMS=%d\n", pos, _total-1, (_total-pos));
   if ( pos <= (_total - 1) ) {	// need to move memory around?