ui/vnc: fix debug output for invalid audio message

The debug message was cut and pasted from the invalid audio format
case, but the audio message is at bytes 2-3.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/ui/vnc.c b/ui/vnc.c
index 6fd8699..1684ab9 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -2551,7 +2551,7 @@
                     vs, vs->ioc, vs->as.fmt, vs->as.nchannels, vs->as.freq);
                 break;
             default:
-                VNC_DEBUG("Invalid audio message %d\n", read_u8(data, 4));
+                VNC_DEBUG("Invalid audio message %d\n", read_u8(data, 2));
                 vnc_client_error(vs);
                 break;
             }