Fix ASN.1 memory management ownership in NGAP functions

In the current memory management the caller wasn't responsible for freeing
memory they allocate. The ngap_gNB_encode_pdu function was taking ownership
of PDU contents by calling ASN_STRUCT_FREE_CONTENTS_ONLY, while callers
also expected to manage the same memory. This created unclear ownership semantics and
potential double-free bugs.

- Remove ASN_STRUCT_FREE_CONTENTS_ONLY from ngap_gNB_encode_pdu to prevent double-free
- Remove unnecessary ASN_STRUCT_FREE when encode function returns NULL
- Add proper ASN_STRUCT_FREE cleanup for heap-allocated PDUs in ngap_gNB.c functions
- Add ASN_STRUCT_FREE_CONTENTS_ONLY cleanup for stack-allocated PDUs in NAS and context management functions
- Ensure consistent memory cleanup patterns across all NGAP encoder callers
- Fix potential memory leaks in all ngap_gNB_encode_pdu call sites

This prevents double-free bugs and ensures proper cleanup of ASN.1 structures
in both heap and stack allocation scenarios.

openair3/NGAP/ngap_gNB_encoder.c

@@ -148,6 +148,5 @@ int ngap_gNB_encode_pdu(NGAP_NGAP_PDU_t *pdu, uint8_t **buffer, uint32_t *len)
       NGAP_DEBUG("Unknown message outcome (%d) or not implemented", (int)pdu->present);
       return -1;
   }
-  ASN_STRUCT_FREE_CONTENTS_ONLY(asn_DEF_NGAP_NGAP_PDU, pdu);
   return ret;
 }