Don't write unused part of the buffer into image data.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/20698


(cherry picked from commit 3aded5da4b)

Co-authored-by: Alex Shvartzkop <dudesuchamazing@gmail.com>

common/bitmap_base.cpp

@@ -208,7 +208,7 @@ bool BITMAP_BASE::SaveImageData( wxOutputStream& aOutStream ) const
     else
     {
         // Write the contents of m_imageData to the stream.
-        aOutStream.Write( m_imageData.GetData(), m_imageData.GetBufSize() );
+        aOutStream.Write( m_imageData.GetData(), m_imageData.GetDataLen() );
     }
 
     return true;