MT#6561 Make sure daemon executable is available before executing any actions

Otherwise we might run into situations like that:

| Removing ngcp-schema ...
| Force-reloading ngcp-panel to handle schema changes
| [....] Restarting web application: ngcp-panel not runningstart-stop-daemon: unable to stat /usr/share/ngcp-panel/ngcp_panel_fastcgi.pl (No such file or directory)
| [FAIL] error ... failed!
|  failed!
| invoke-rc.d: initscript ngcp-panel, action "force-reload" failed.

debian/ngcp-panel.init

@@ -27,6 +27,11 @@ NPROC=1
 
 OPTIONS="--listen $USOCKET --daemon --pidfile $PIDFILE --nproc $NPROC"
 
+if ! [ -x "$DAEMON" ] ; then
+  log_warning_msg "File $DAEMON not available/executable."
+  exit 0
+fi
+
 check_running() {
   if [ -s $PIDFILE ]; then
     kill -0 $(cat $PIDFILE) >/dev/null 2>&1