nbis: Fix crash in gen_initial_maps()
nbis: Fix crash in gen_initial_maps()

If the image is too small some coordinates can become negative.
Handle this to avoid memory corruption.

https://bugs.freedesktop.org/show_bug.cgi?id=57730

jipumarino/libfprint,ccsystems/libfprint,ccsystems/libfprint,yingted/etes603,hyperair/fprint-bsapi,freedesktop-unofficial-mirror/libfprint__libfprint,zemen/libfprint,iafilatov/libfprint,Gnate/libfprint,freedesktop-unofficial-mirror/libfprint__libfprint,PDXostc/libfprint,zOrg1331/libfprint,rmarku/libfprint,hyperair/fprint-bsapi,abhishekgahlot/libfprint,zemen/libfprint,micw/libfprint,Gnate/libfprint,jipumarino/libfprint,yingted/etes603,abhishekgahlot/fprint-bsapi,anarsoul/libfprint,rmarku/libfprint,Gnate/libfprint,sbambrough/libfprint_extended,abhishekgahlot/libfprint,zOrg1331/libfprint,FabriceSalvaire/libfprint-baspi,anarsoul/libfprint,ccsystems/libfprint,payden/libfprint,zemen/libfprint,micw/libfprint,zOrg1331/libfprint,payden/libfprint,freedesktop-unofficial-mirror/libfprint__libfprint,abhishekgahlot/fprint-bsapi,hyperair/fprint-bsapi,abhishekgahlot/fprint-bsapi,abhishekgahlot/libfprint,sbambrough/libfprint_extended,PDXostc/libfprint,jipumarino/libfprint,PDXostc/libfprint,sbambrough/libfprint_extended,rmarku/libfprint,FabriceSalvaire/libfprint-baspi,micw/libfprint,FabriceSalvaire/libfprint-baspi,payden/libfprint,yingted/etes603