Fix garbage in PNG files

Doubtful that this itself is a security flaw as 1-2 bytes is probably not sufficient to hide anything, but probably ought to fix them anyways.

debian/osiris/opt/osiris/share/osiris/doc/html/pnglogo.png: 1 bytes of garbage after IEND chunk at /usr/share/perl5/File/StripNondeterminism/handlers/png.pm line 172.
debian/osiris/opt/osiris/share/osiris/doc/html/gridlayout.png: 1 bytes of garbage after IEND chunk at /usr/share/perl5/File/StripNondeterminism/handlers/png.pm line 172.
debian/osiris/opt/osiris/share/osiris/doc/html/whatsthis.png: 1 bytes of garbage after IEND chunk at /usr/share/perl5/File/StripNondeterminism/handlers/png.pm line 172.
debian/osiris/opt/osiris/share/osiris/doc/html/test.png: 2 bytes of garbage after IEND chunk at /usr/share/perl5/File/StripNondeterminism/handlers/png.pm line 172.
debian/osiris/opt/osiris/share/osiris/doc/html/palette.png: 2 bytes of garbage after IEND chunk at /usr/share/perl5/File/StripNondeterminism/handlers/png.pm line 172.
debian/osiris/opt/osiris/share/osiris/doc/html/dochead.png: 2 bytes of garbage after IEND chunk at /usr/share/perl5/File/StripNondeterminism/handlers/png.pm line 172.

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