[client] core: don't try to send LGMP messages if the video is stopped

If the video stream is stopped the LGMP session is not valid, so we
can't send messages to the client.

client/src/core.c

@@ -448,7 +448,8 @@ void core_handleMouseNormal(double ex, double ey)
     struct DoublePoint guest;
     util_localCurToGuest(&guest);
 
-    if (g_state.kvmfrFeatures & KVMFR_FEATURE_SETCURSORPOS)
+    if (!g_state.stopVideo &&
+      g_state.kvmfrFeatures & KVMFR_FEATURE_SETCURSORPOS)
     {
       const KVMFRSetCursorPos msg = {
         .msg.type = KVMFR_MESSAGE_SETCURSORPOS,