Bug#856444: gdk-pixbuf: CVE-2017-6312: Possible out-of-bounds read

[Salvatore Bonaccorso]

Control: severity -1 serious
Control: fixed -1 2.36.5-2+deb9u2

This has been fixed in a DSA, but not yet in unstable. Raising
severity to RC to not have a regression stretch->buster.

Regards,
Salvatore