fix invalid neighbor cell addition

if phy measurements are received and processes before a serving cell
is selected, an invalid neighbor cell is added because the uninitlized
serving cell is used.

fix the issue by checking if the serving cell is valid

srsue/src/stack/rrc/rrc.cc

@@ -686,12 +686,16 @@ bool rrc::add_neighbour_cell(cell_t *new_cell) {
 }
 
 // If only neighbour PCI is provided, copy full cell from serving cell
-bool rrc::add_neighbour_cell(uint32_t earfcn, uint32_t pci, float rsrp) {
+bool rrc::add_neighbour_cell(uint32_t earfcn, uint32_t pci, float rsrp)
+{
+  if (serving_cell->is_valid()) {
     phy_interface_rrc_lte::phy_cell_t phy_cell;
     phy_cell         = serving_cell->phy_cell;
     phy_cell.earfcn  = earfcn;
     phy_cell.cell.id = pci;
     return add_neighbour_cell(phy_cell, rsrp);
+  }
+  return false;
 }
 
 bool rrc::add_neighbour_cell(phy_interface_rrc_lte::phy_cell_t phy_cell, float rsrp)