enb,phy: do not start start plots if LTE workers don't exist

this was causing a segfault when the GUI was enabled in SA mode, i.e. without LTE cells
we only support the GUI with LTE enabled.

srsenb/src/phy/phy.cc

@@ -349,7 +349,9 @@ void phy::configure_mbsfn(srsran::sib2_mbms_t* sib2, srsran::sib13_t* sib13, con
 // Start GUI
 void phy::start_plot()
 {
-  lte_workers[0]->start_plot();
+  if (lte_workers.get_nof_workers() > 0) {
+    lte_workers[0]->start_plot();
+  }
 }
 
 int phy::init_nr(const phy_args_t& args, const phy_cfg_t& cfg, stack_interface_phy_nr& stack)