MDEV-27451 gcol.virtual_index_drop fails with LeakSanitizer errors

Because commit 24773bf
made dict_v_col_t encapsulate v_indexes, we must invoke
dict_v_col_t::~dict_v_col_t() to destruct the container.

This basically is a fixup of the merge
commit 5008171
of the 10.2
commit cf2c6b7 (MDEV-24971).

I did not debug why no leaks are reported for 10.2 or 10.3.

Author: dr-m

storage/innobase/handler/handler0alter.cc

@@ -8914,6 +8914,14 @@ rollback_inplace_alter_table(
 
 			row_mysql_unlock_data_dictionary(prebuilt->trx);
 		}
+
+		if (ctx->add_vcol) {
+			for (ulint i = 0; i < ctx->num_to_add_vcol; i++) {
+				ctx->add_vcol[i].~dict_v_col_t();
+			}
+			ctx->num_to_add_vcol = 0;
+			ctx->add_vcol = nullptr;
+		}
 	}
 
 	/* Reset dict_col_t::ord_part for those columns fail to be indexed,