Handle empty rects from the server

These are very pointless for the server to send, but not a violation of
the protocol so we need to be able to handle them. We've seen this
happen in real world scenarios a few times.

kasmweb/core/decoders/copyrect.js

@@ -15,6 +15,11 @@ export default class CopyRectDecoder {
 
         let deltaX = sock.rQshift16();
         let deltaY = sock.rQshift16();
+
+        if ((width === 0) || (height === 0)) {
+            return true;
+        }
+
         display.copyImage(deltaX, deltaY, x, y, width, height);
 
         return true;

kasmweb/core/decoders/tight.js

@@ -162,6 +162,10 @@ export default class TightDecoder {
         const uncompressedSize = width * height * 3;
         let data;
 
+        if (uncompressedSize === 0) {
+            return true;
+        }
+
         if (uncompressedSize < 12) {
             if (sock.rQwait("TIGHT", uncompressedSize)) {
                 return false;
@@ -217,6 +221,10 @@ export default class TightDecoder {
 
         let data;
 
+        if (uncompressedSize === 0) {
+            return true;
+        }
+
         if (uncompressedSize < 12) {
             if (sock.rQwait("TIGHT", uncompressedSize)) {
                 return false;