Fix duplicate menu at end of buffer (#937)

Author: parkovski

PSReadLine/Completion.cs

@@ -471,9 +471,12 @@ public void DrawMenu(Menu previousMenu, bool menuSelect)
                 // Move cursor to the start of the first line after our input.
                 var bufferEndPoint = Singleton.ConvertOffsetToPoint(Singleton._buffer.Length);
                 console.SetCursorPosition(bufferEndPoint.X, bufferEndPoint.Y);
+                // Top must be initialized before calling AdjustForPossibleScroll, otherwise
+                // on the last line of the buffer, the scroll operation causes Top to point
+                // past the buffer, which in turn causes the menu to be printed twice.
+                this.Top = bufferEndPoint.Y + 1;
                 AdjustForPossibleScroll(1);
                 MoveCursorDown(1);
-                this.Top = bufferEndPoint.Y + 1;
 
                 var bufferWidth = console.BufferWidth;
                 var columnWidth = this.ColumnWidth;