Copy dropped data to wxDataViewEvent

Existing code relies on GetDataBuffer() returning a valid pointer, so we
need to ensure that this is the case, even if this means copying the
data into an internal buffer.

include/wx/dataview.h

@@ -972,6 +972,7 @@ protected:
 #if wxUSE_DRAG_AND_DROP
     wxDataObject       *m_dataObject;
 
+    wxMemoryBuffer      m_dataBuf;
     wxDataFormat        m_dataFormat;
     void*               m_dataBuffer;
     size_t              m_dataSize;

src/common/datavcmn.cpp

@@ -1812,7 +1812,17 @@ void wxDataViewEvent::InitData(wxDataObjectComposite* obj, wxDataFormat format)
     SetDataFormat(format);
 
     SetDataObject(obj->GetObject(format));
-    SetDataSize(obj->GetDataSize(format));
+
+    const size_t size = obj->GetDataSize(format);
+    SetDataSize(size);
+
+    if ( size )
+    {
+        obj->GetDataHere(format, m_dataBuf.GetWriteBuf(size));
+        m_dataBuf.UngetWriteBuf(size);
+
+        SetDataBuffer(m_dataBuf.GetData());
+    }
 }
 
 #endif // wxUSE_DRAG_AND_DROP