Refuse to start Xvnc if we failed to initialise a screen

Starting Xvnc without having any VNC functionality is pretty much
pointless. So terminate when that happens, making the situation easier
to detect for startup scripts.

unix/xserver/hw/vnc/vncExtInit.cc

@@ -205,6 +205,11 @@ void vncExtensionInit(void)
   vncRegisterBlockHandlers();
 }
 
+int vncExtensionIsActive(int scrIdx)
+{
+  return (desktop[scrIdx] != NULL);
+}
+
 void vncCallReadBlockHandlers(fd_set * fds, struct timeval ** timeout)
 {
   for (int scr = 0; scr < vncGetScreenCount(); scr++)

unix/xserver/hw/vnc/vncExtInit.h

@@ -50,6 +50,7 @@ extern int vncFbstride[];
 extern int vncInetdSock;
 
 void vncExtensionInit(void);
+int vncExtensionIsActive(int scrIdx);
 
 void vncCallReadBlockHandlers(fd_set * fds, struct timeval ** timeout);
 void vncCallReadWakeupHandlers(fd_set * fds, int nfds);

unix/xserver/hw/vnc/xvnc.c

@@ -1744,6 +1744,11 @@ void ProcessInputEvents(void)
 
 void InitInput(int argc, char *argv[])
 {
+  int i;
+  for (i = 0;i < screenInfo.numScreens;i++) {
+    if (!vncExtensionIsActive(i))
+        FatalError("failed to activate VNC extension for one or more screens");
+  }
   mieqInit ();
 }