Return false, not NULL, from a function returning bool.

Fix a warning (which might be an error with some compilers) about returning
NULL from bool wxRichTextImageBlock::DoMakeImageBlock() function.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66376 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/richtext/richtextbuffer.cpp

@@ -7844,7 +7844,7 @@ bool wxRichTextImageBlock::DoMakeImageBlock(const wxImage& image, wxBitmapType i
 
     unsigned char* block = new unsigned char[memStream.GetSize()];
     if (!block)
-        return NULL;
+        return false;
 
     if (m_data)
         delete[] m_data;