Overview: ignore alpha=0 values when compute an average overview of an alpha band; and also avoid memory errors when calling GetMaskBand()/GetMaskFlags() after overview computation if GetMaskXXX() has been called before (#5640)

git-svn-id: https://svn.osgeo.org/gdal/branches/1.11@27657 f0d54148-0727-0410-94bb-9a71ac55c965