JPEG: avoid memory leak when GDALOpen'ing() a JPEG through a http:// URL, and make it possible to access its overviews

git-svn-id: https://svn.osgeo.org/gdal/trunk@26352 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx