avoid loss of text in edited placeholder

How to reproduce:
1. Create an empty presentation.
2. Save it.
3. Click at one of the text placeholders and write something.
4. Save again.
5. Reload. The placeholder is missing. (Actually, it's still there, but
   empty and 0-size.)

This only happens if a11y is enabled.

Change-Id: I7ae3d3c6ab0ce91aef01c0e9d62f7963dce965fc

svx/source/unodraw/unoshtxt.cxx

@@ -589,6 +589,7 @@ SvxTextForwarder* SvxTextEditSourceImpl::GetBackgroundTextForwarder()
         bCreated = true;
 
         mbForwarderIsEditMode = false;
+        mbDataValid = false;
     }
 
     if( mpObject && mpText && !mbDataValid && mpObject->IsInserted() && mpObject->GetPage() )