In some corner cases (that never happen during normal operation but a
malicious guest could program wrong values) pixman functions were
called with parameters that result in a crash. Fix this and add more
checks to disallow such cases.

Reported-by: Ziming Zhang <ezrakiez@gmail.com>
Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Message-id: 20200406204029.19559747D5D@zero.eik.bme.hu
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>