vnc: fix tight png memory leak

The tight.png buffer was never released.
Signed-off-by: NCorentin Chary <corentincj@iksaif.net>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

ui/vnc-enc-tight.c

@@ -1674,4 +1674,7 @@ void vnc_tight_clear(VncState *vs)
 #ifdef CONFIG_VNC_JPEG
     buffer_free(&vs->tight.jpeg);
 #endif
+#ifdef CONFIG_VNC_PNG
+    buffer_free(&vs->tight.png);
+#endif
 }