Fix memory leak in dlInformationTransfer case

- Remove unnecessary NULL assignment for dedicatedNAS_Message->buf
- The original buffer is copied and transfered to NAS, so it can be properly freed by ASN.1 structure cleanup

Root cause: Code was nullifying dedicatedNAS_Message->buf pointer before ASN.1 cleanup could free the original allocation, causing the leak from OCTET_STRING_decode_uper.

AddressSanitizer leak trace (FIXED):
Direct leak of 119 byte(s) in 3 object(s) allocated from:
    /#0 0x7ec2a42b4c38 in __interceptor_realloc
    /#1 0x555fade852e0 in OCTET_STRING_decode_uper
    /#9 0x555fadb48b11 in nr_rrc_ue_decode_dcch.

openair2/RRC/NR_UE/rrc_UE.c

@@ -2299,7 +2299,6 @@ static int nr_rrc_ue_decode_dcch(NR_UE_RRC_INST_t *rrc,
               msg->nasMsg.nas_data = malloc(msg->nasMsg.length);
               memcpy(msg->nasMsg.nas_data, dedicatedNAS_Message->buf, msg->nasMsg.length);
               itti_send_msg_to_task(TASK_NAS_NRUE, rrc->ue_id, ittiMsg);
-              dedicatedNAS_Message->buf = NULL; // to keep the buffer, up to NAS to free it
             }
           }
         } break;