spapr-vty: Fix bad assert() statement

spapr-vty: Fix bad assert() statement

When using the serial console in the GTK interface of QEMU (and
QEMU has been compiled with CONFIG_VTE), it is possible to trigger
the assert() statement in vty_receive() in spapr_vty.c by pasting
a chunk of text with length > 16 into the QEMU window.
Most of the other serial backends seem to simply drop characters
that they can not handle, so I think we should also do the same in
spapr-vty to fix this issue.

Buglink: https://bugs.launchpad.net/qemu/+bug/1639322
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>