[gtk-vnc] src: correctly handle zero size alpha cursor

[Daniel P. Berrange <dberrange src gnome org>]

commit a818f5c3444cd3fcd462c45254c4ab78815ce431
Author: Daniel P. Berrangé <dan berrange com>
Date:   Fri Dec 11 15:43:31 2020 +0000

    src: correctly handle zero size alpha cursor
    
    Even with a zero size cursor we still have to read the encoding type off
    the wire. Assuming we get a raw encoding, we don't have any further data
    to read.
    
    Signed-off-by: Daniel P. Berrangé <berrange redhat com>

 src/vncconnection.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
---
diff --git a/src/vncconnection.c b/src/vncconnection.c
index a2aec30..6d34c5a 100644
--- a/src/vncconnection.c
+++ b/src/vncconnection.c
@@ -3208,6 +3208,20 @@ static void vnc_connection_alpha_cursor(VncConnection *conn, guint16 x, guint16
                                       x, y, width, height);
         /* pixels is owned by priv->cursor now */
         g_object_unref(fb);
+    } else {
+        guint32 encoding = vnc_connection_read_u32(conn);
+        if (vnc_connection_has_error(conn)) {
+            return;
+        }
+        switch (encoding) {
+        case VNC_CONNECTION_ENCODING_RAW:
+            break;
+        default:
+            vnc_connection_set_error(conn,
+                                     "Unsupported encoding %u for zero size alpha cursor",
+                                     encoding);
+            break;
+        }
     }
 
     sigdata.params.cursor = priv->cursor;