replace broken debian-logo.png with proper version

The rendering of the logo this replaces can be recognised as faulty by the fact that the leftmost edge, at the 9 o'clock position, is flattened and as the swirl sweeps down towards the outer tail, the line broadens much more than the correct logo, making the 8 o'clock position as thick as the 12 o'clock position (whereas it should be relatively narrow).

This replacement image was created in Inkscape by taking: https://www.debian.org/logos/openlogo-nd.svg and exporting is as a 45px tall PNG which was then placed using GIMP into a 48x48 image, and again exported as PNG.