check configured flag of SCell struct before enabling SCell

this prevents crashing the UE when receiving a SCell activation
command before the SCell has been configured from RRC

srsue/src/phy/phy_common.cc

@@ -847,7 +847,13 @@ bool phy_common::is_mbsfn_sf(srslte_mbsfn_cfg_t* cfg, uint32_t phy_tti)
 void phy_common::enable_scell(uint32_t cc_idx, bool enable)
 {
   if (cc_idx < SRSLTE_MAX_CARRIERS) {
+    if (scell_cfg[cc_idx].configured) {
       scell_cfg[cc_idx].enabled = enable;
+    } else {
+      Error("Leaving SCell %s. cc_idx=%d has not been configured.\n",
+            scell_cfg[cc_idx].enabled ? "enabled" : "disabled",
+            cc_idx);
+    }
   }
 }