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>

ui/vnc-enc-tight.c

@@ -1546,7 +1546,7 @@ static int send_sub_rect(VncState *vs, int x, int y, int w, int h)
     vnc_tight_stop(vs);
 
 #ifdef CONFIG_VNC_JPEG
-    if (vs->tight.quality != -1) {
+    if (vs->tight.quality != (uint8_t)-1) {
         double freq = vnc_update_freq(vs, x, y, w, h);
 
         if (freq < tight_jpeg_conf[vs->tight.quality].jpeg_freq_min) {
@@ -1711,7 +1711,7 @@ static int tight_send_framebuffer_update(VncState *vs, int x, int y,
         vs->tight.pixel24 = false;
     }
 
-    if (vs->tight.quality != -1) {
+    if (vs->tight.quality != (uint8_t)-1) {
         double freq = vnc_update_freq(vs, x, y, w, h);
 
         if (freq > tight_jpeg_conf[vs->tight.quality].jpeg_freq_threshold) {

ui/vnc-enc-zrle.c

@@ -260,7 +260,8 @@ static int zrle_send_framebuffer_update(VncState *vs, int x, int y,
     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) {