Fix integer overflow in MallocFrameBuffer()

Promote integers to uint64_t to avoid integer overflow issue during
frame buffer allocation for very large screen sizes

libvncclient/vncviewer.c

@@ -82,9 +82,27 @@ static char* ReadPassword(rfbClient* client) {
 #endif
 }
 static rfbBool MallocFrameBuffer(rfbClient* client) {
+uint64_t allocSize;
+
   if(client->frameBuffer)
     free(client->frameBuffer);
-  client->frameBuffer=malloc(client->width*client->height*client->format.bitsPerPixel/8);
+
+  /* SECURITY: promote 'width' into uint64_t so that the multiplication does not overflow
+     'width' and 'height' are 16-bit integers per RFB protocol design
+     SIZE_MAX is the maximum value that can fit into size_t
+  */
+  allocSize = (uint64_t)client->width * client->height * client->format.bitsPerPixel/8;
+
+  if (allocSize >= SIZE_MAX) {
+    rfbClientErr("CRITICAL: cannot allocate frameBuffer, requested size is too large\n");
+    return FALSE;
+  }
+
+  client->frameBuffer=malloc( (size_t)allocSize );
+
+  if (client->frameBuffer == NULL)
+    rfbClientErr("CRITICAL: frameBuffer allocation failed, requested size too large or not enough memory?\n");
+
   return client->frameBuffer?TRUE:FALSE;
 }