[fix](storage-vectorized) fix VMergeIterator core dump (#9564)

It could be re appeared on rowset with many segment, it means segment overlap. Maybe could not reappear it easily.

be/src/vec/olap/vgeneric_iterators.cpp

@@ -178,9 +178,7 @@ class VMergeIteratorContext {
         vectorized::Block& src = _block;
         vectorized::Block& dst = *block;
 
-        auto columns = _iter->schema().columns();
-
-        for (size_t i = 0; i < columns.size(); ++i) {
+        for (size_t i = 0; i < _iter->schema().num_column_ids(); ++i) {
             vectorized::ColumnWithTypeAndName s_col = src.get_by_position(i);
             vectorized::ColumnWithTypeAndName d_col = dst.get_by_position(i);