Fix calculation of the wxStyledTextCtrl selection size.

Use SCI_GETSELTEXT to compute the size of the buffer instead of doing it
ourselves, especially as we do it incorrectly in case of rectangular
selection.

Closes #14331.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71540 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/stc/stc.cpp

@@ -4128,16 +4128,10 @@ wxCharBuffer wxStyledTextCtrl::GetLineRaw(int line)
 
 wxCharBuffer wxStyledTextCtrl::GetSelectedTextRaw()
 {
-    long   start;
-    long   end;
-
-    GetSelection(&start, &end);
-    int   len  = end - start;
-    if (!len) {
-        wxCharBuffer empty;
-        return empty;
-    }
+    // Calculate the length needed first.
+    const int len = SendMsg(SCI_GETSELTEXT, 0, (sptr_t)0);
 
+    // And then really get the data.
     wxCharBuffer buf(len);
     SendMsg(SCI_GETSELTEXT, 0, (sptr_t)buf.data());
     return buf;