Cope gracefully with lack of default_witness_commitment despite rules supporting segwit.

src/stratifier.c

@@ -1449,8 +1449,11 @@ retry:
 				witnessdata_check = json_string_value(json_object_get(wb->json, "default_witness_commitment"));
 				gbt_witness_data(wb, txn_array);
 				// Verify against the pre-calculated value if it exists. Skip the size/OP_RETURN bytes.
+				if (likely(witnessdata_check)) {
 					if (wb->insert_witness && witnessdata_check[0] && safecmp(witnessdata_check + 4, wb->witnessdata) != 0)
 						LOGERR("Witness from btcd: %s. Calculated Witness: %s", witnessdata_check + 4, wb->witnessdata);
+				} else
+					LOGNOTICE("Segwit rules returned but no default_witness_commitment to check witness data");
 				break;
 			}
 		}