Fix wxStyledTextCtrl::SetInsertionPointEnd().

This relied on SetInsertionPoint(-1) working like in wxTextCtrl but it didn't.
Add an explicit check to it for consistency with the other classes and to make
SetInsertionPointEnd() work.

Closes #15234.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74069 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/stc/stc.h

@@ -4577,7 +4577,10 @@ public:
 
     */
 
-    virtual void SetInsertionPoint(long pos) { SetCurrentPos(pos); }
+    virtual void SetInsertionPoint(long pos)
+    {
+        SetCurrentPos(pos == -1 ? GetLastPosition() : pos);
+    }
     virtual long GetInsertionPoint() const { return GetCurrentPos(); }
     virtual long GetLastPosition() const { return GetTextLength(); }