vnc: fix uint8_t comparisons with negative values

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/ui/vnc-enc-zrle.c b/ui/vnc-enc-zrle.c
index 016a406..917d384 100644
--- a/ui/vnc-enc-zrle.c
+++ b/ui/vnc-enc-zrle.c
@@ -260,7 +260,8 @@
     int zywrle_level;
 
     if (vs->zrle.type == VNC_ENCODING_ZYWRLE) {
-        if (!vs->vd->lossy || vs->tight.quality < 0 || vs->tight.quality == 9) {
+        if (!vs->vd->lossy || vs->tight.quality == (uint8_t)-1
+            || vs->tight.quality == 9) {
             zywrle_level = 0;
             vs->zrle.type = VNC_ENCODING_ZRLE;
         } else if (vs->tight.quality < 3) {