SRSUE: Protect nullptr buffer in secondary serving cell synch

srsue/hdr/phy/scell/scell_sync.h

@@ -71,7 +71,11 @@ private:
     uint32_t peak_pos = 0;
 
     // Append new base-band
+    if (buffer == nullptr) {
+      srslte_vec_cf_zero(&temp[sf_len], sf_len);
+    } else {
       srslte_vec_cf_copy(&temp[sf_len], buffer, sf_len);
+    }
 
     // Run PSS search
     switch (srslte_sync_find(&find_pss, temp.data(), 0, &peak_pos)) {