XtraDB/InnoDB crash with autoinc, vcol and online alter

Fix the doubly questional fix for MySQL Bug#17250787:
* it detected autoinc index by looking for the first index
  that starts from autoinc column. never mind one column
  can be part of many indexes.
* it used autoinc_field->field_index to look up into internal
  innodb dictionary. But field_index accounts for virtual
  columns too, while innodb dictionary ignores them.

Find the index by its name, like elsewhere in ha_innobase.

mysql-test/suite/vcol/r/innodb_autoinc_vcol.result

+create table t1 (c2 int as (-c1), c1 int primary key auto_increment) engine=innodb;
+insert into t1(c1) values (null),(null),(null);
+select * from t1;
+c2	c1
+-1	1
+-2	2
+-3	3
+alter table t1 auto_increment = 3;
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `c2` int(11) AS (-c1) VIRTUAL,
+  `c1` int(11) NOT NULL AUTO_INCREMENT,
+  PRIMARY KEY (`c1`)
+) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=latin1
+drop table t1;

mysql-test/suite/vcol/t/innodb_autoinc_vcol.test

+--source include/have_innodb.inc
+
+create table t1 (c2 int as (-c1), c1 int primary key auto_increment) engine=innodb;
+insert into t1(c1) values (null),(null),(null);
+select * from t1;
+alter table t1 auto_increment = 3;
+show create table t1;
+drop table t1;
+

storage/innobase/handler/handler0alter.cc

@@ -4736,9 +4736,11 @@ commit_get_autoinc(
 
 		Field*	autoinc_field =
 			old_table->found_next_number_field;
+		KEY*	autoinc_key =
+			old_table->key_info + old_table->s->next_number_index;
 
-		dict_index_t*	index = dict_table_get_index_on_first_col(
-			ctx->old_table, autoinc_field->field_index);
+		dict_index_t*	index = dict_table_get_index_on_name(
+			ctx->old_table, autoinc_key->name);
 
 		max_autoinc = ha_alter_info->create_info->auto_increment_value;
 

storage/xtradb/handler/handler0alter.cc

@@ -4743,9 +4743,11 @@ commit_get_autoinc(
 
 		Field*	autoinc_field =
 			old_table->found_next_number_field;
+		KEY*	autoinc_key =
+			old_table->key_info + old_table->s->next_number_index;
 
-		dict_index_t*	index = dict_table_get_index_on_first_col(
-			ctx->old_table, autoinc_field->field_index);
+		dict_index_t*	index = dict_table_get_index_on_name(
+			ctx->old_table, autoinc_key->name);
 
 		max_autoinc = ha_alter_info->create_info->auto_increment_value;