Fix vnc memory corruption with width = 1400

vnc assumes that the screen width is a multiple of 16 in several places.
If this is not the case vnc will overrun buffers, corrupt memory, make
qemu crash.

This is the minimum fix for this bug. It makes sure we don't overrun the
scanline, thereby fixing the segfault.  The rendering is *not* correct
though, there is a black border at the right side of the screen, 8
pixels wide because 1400 % 16 == 8.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
