Account for the last position in wxMSW wxTextCtrl

There is a valid position after the last character of the text in
wxTextCtrl, e.g. position 0 in the empty control, so account for it and,
notably, don't return -1 from XYToPosition(0, 0) when the control is
empty.

This fixes a regression in a69ab2907c.

src/msw/textctrl.cpp

@@ -1486,13 +1486,12 @@ long wxTextCtrl::XYToPosition(long x, long y) const
 
     // Line is identified by a character position!
     long lineLength = ::SendMessage(GetHwnd(), EM_LINELENGTH, charIndex, 0);
-    // For all lines but last one we need to adjust the length
-    // to include new line character (only one because both CR and LF
-    // are virtually "displayed" at the same position).
-    if ( y < GetNumberOfLines() - 1 )
-        lineLength += 1;
 
-    if ( x >= lineLength )
+    // Notice that x == lineLength is still valid because it corresponds either
+    // to the position of the LF at the end of any line except the last one or
+    // to the last position, which is the position after the last character,
+    // for the last line.
+    if ( x > lineLength )
         return -1;
 
     return charIndex + x;