Account for the last position in wxTextCtrl (wxOSX)

Take into account that last valid position is after the last character of the text.
This applies also to empty controls for which XYToPosition(0, 0) should return 0.

src/osx/cocoa/textctrl.mm

@@ -915,14 +915,24 @@ long wxNSTextViewControl::XYToPosition(long x, long y) const
 
     NSString* txt = [m_textView string];
     const long txtLen = [txt length];
-    long nline = 0;
+    long nline = -1;
     NSRange lineRng;
-    for ( long i = 0; i < txtLen && nline <= y; nline++ )
+    long i = 0;
+    // We need to enter the counting loop at least once,
+    // even for empty text string.
+    do
     {
+        nline++;
         lineRng = [txt lineRangeForRange:NSMakeRange(i, 0)];
         i = NSMaxRange(lineRng);
-    }
-    nline--;
+    } while ( i < txtLen && nline < y );
+    // In the last line, the last valid position is after
+    // the last real character, so we need to extended actual
+    // range to count this additional virtual character.
+    // In any other line, the last valid position is at
+    // the new line character which is already counted.
+    if ( i == txtLen )
+        lineRng.length++;
 
     // Return error if contol contains
     // less lines than given y position.
@@ -1276,7 +1286,8 @@ long wxNSTextFieldControl::XYToPosition(long x, long y) const
 {
     wxCHECK_MSG( x >= 0 && y >= 0, -1, wxS("Invalid line/column number") );
 
-    if ( y != 0 || x >= [[m_textField stringValue] length] )
+    // Last valid position is after the last character.
+    if ( y != 0 || x > [[m_textField stringValue] length] )
         return -1;
 
     return x;