vnc: Fix tight_detect_smooth_image() for lossless case

VncTight member uint8_t quality is either (uint8_t)-1 for lossless or
less than 10 for lossy.

tight_detect_smooth_image() first promotes it to int, then compares
with -1.  Always unequal, so we always execute the lossy code.  Reads
beyond tight_conf[] and returns crap when quality is actually
lossless.

Compare to (uint8_t)-1 instead, like we do elsewhere.

Spotted by Coverity.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

ui/vnc-enc-tight.c

@@ -330,7 +330,7 @@ tight_detect_smooth_image(VncState *vs, int w, int h)
     } else {
         errors = tight_detect_smooth_image16(vs, w, h);
     }
-    if (quality != -1) {
+    if (quality != (uint8_t)-1) {
         return (errors < tight_conf[quality].jpeg_threshold);
     }
     return (errors < tight_conf[compression].gradient_threshold);