Fix a segfault when processing input to empty FListView

If the application has a list view with no items and the user clicks the widget
or sends any key (space, plus, minus etc) which acts on the items, the
application will segfault as the item returned from `getCurrentItem()` is
`nullptr` and there's no check made for this condition.

Instead of checking whether current item `!= nullptr` just check whether the
item list is empty and avoid running any code at all in such case.

src/flistview.cpp

@@ -995,6 +995,9 @@ void FListView::onMouseDown (FMouseEvent* ev)
     }
     else if ( mouse_y > 1 && mouse_y < int(getHeight()) )  // List
     {
+      if ( itemlist.empty () )
+        return;
+
       int indent = 0;
       int new_pos = first_visible_line.getPosition() + mouse_y - 2;
 
@@ -1057,6 +1060,9 @@ void FListView::onMouseUp (FMouseEvent* ev)
       }
       else if ( mouse_y > 1 && mouse_y < int(getHeight()) )  // List
       {
+        if (itemlist.empty ())
+          return;
+
         int indent = 0;
         auto item = getCurrentItem();
 
@@ -1164,6 +1170,9 @@ void FListView::onMouseDoubleClick (FMouseEvent* ev)
     if ( first_visible_line.getPosition() + mouse_y - 1 > int(getCount()) )
       return;
 
+    if ( itemlist.empty () )
+      return;
+
     auto item = getCurrentItem();
 
     if ( tree_view && item->isExpandable() )
@@ -2184,6 +2193,9 @@ void FListView::processChanged()
 //----------------------------------------------------------------------
 inline void FListView::keySpace()
 {
+  if ( itemlist.empty () )
+    return;
+
   auto item = getCurrentItem();
 
   if ( item->isCheckable() )
@@ -2193,6 +2205,9 @@ inline void FListView::keySpace()
 //----------------------------------------------------------------------
 inline void FListView::keyLeft (int& first_line_position_before)
 {
+  if ( itemlist.empty () )
+    return;
+
   int position_before = current_iter.getPosition();
   auto item = getCurrentItem();
 
@@ -2247,6 +2262,9 @@ inline void FListView::keyLeft (int& first_line_position_before)
 //----------------------------------------------------------------------
 inline void FListView::keyRight (int& first_line_position_before)
 {
+  if ( itemlist.empty () )
+    return;
+
   int xoffset_end = int(max_line_width) - int(getClientWidth());
   auto item = getCurrentItem();
 
@@ -2291,6 +2309,9 @@ inline void FListView::keyEnd()
 //----------------------------------------------------------------------
 inline bool FListView::keyPlus()
 {
+  if ( itemlist.empty () )
+    return false;
+
   auto item = getCurrentItem();
 
   if ( tree_view && item->isExpandable() && ! item->isExpand() )
@@ -2306,6 +2327,9 @@ inline bool FListView::keyPlus()
 //----------------------------------------------------------------------
 inline bool FListView::keyMinus()
 {
+  if ( itemlist.empty () )
+    return false;
+
   auto item = getCurrentItem();
 
   if ( tree_view && item->isExpandable() && item->isExpand() )