Fix NULL derefencing in columnar_result_set_row (#9762)

Fix Coverity report issue:

    >>>     CID 503639:           (FORWARD_NULL)
>>> Passing "&columnar_result" to "columnar_result_set_row", which
dereferences null "columnar_result.offset_buffer".
577 columnar_result_set_row(&columnar_result, batch_state, row, result,
isnull);

Disable-check: force-changelog-file
(cherry picked from commit d4c12e2)

Author: natalya-aksman

tsl/src/nodes/vector_agg/exec.c

@@ -194,6 +194,8 @@ columnar_result_set_row(ColumnarResult *columnar_result, DecompressBatchState co
 			memcpy(&columnar_result->body_buffer[columnar_result->current_offset],
 				   VARDATA_ANY(datum),
 				   result_bytes);
+			/* offset_buffer should be allocated for ArrowText type */
+			Assert(columnar_result->offset_buffer != NULL);
 			columnar_result->offset_buffer[row] = columnar_result->current_offset;
 			columnar_result->current_offset += result_bytes;
 			break;