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>

hw/char/spapr_vty.c

 #include "qemu/osdep.h"
+#include "qemu/error-report.h"
 #include "qapi/error.h"
 #include "qemu-common.h"
 #include "cpu.h"
@@ -37,7 +38,15 @@ static void vty_receive(void *opaque, const uint8_t *buf, int size)
         qemu_irq_pulse(spapr_vio_qirq(&dev->sdev));
     }
     for (i = 0; i < size; i++) {
-        assert((dev->in - dev->out) < VTERM_BUFSIZE);
+        if (dev->in - dev->out >= VTERM_BUFSIZE) {
+            static bool reported;
+            if (!reported) {
+                error_report("VTY input buffer exhausted - characters dropped."
+                             " (input size = %i)", size);
+                reported = true;
+            }
+            break;
+        }
         dev->buf[dev->in++ % VTERM_BUFSIZE] = buf[i];
     }
 }