Fix and test case for BUG#3899

mysql-test/r/alter_table.result:
  Test case for BUG#3899
mysql-test/t/alter_table.test:
  Test case for BUG#3899
sql/sql_yacc.yy:
  Fix for BUG#3899: call ALTER_INFO::reset to clear ALTER_INFO::drop_list and ALTER_INFO::alter_list

mysql-test/r/alter_table.result

@@ -470,3 +470,9 @@ t1	CREATE TABLE `t1` (
 ALTER TABLE t1 DROP PRIMARY KEY;
 ERROR 42000: Can't DROP 'PRIMARY'; check that column/key exists
 DROP TABLE t1;
+create table t1 (a int, b int, key(a));
+insert into t1 values (1,1), (2,2);
+alter table t1 drop key no_such_key;
+ERROR 42000: Can't DROP 'no_such_key'; check that column/key exists
+alter table t1 drop key a;
+drop table t1;

mysql-test/t/alter_table.test

@@ -304,3 +304,11 @@ SHOW CREATE TABLE t1;
 --error 1091
 ALTER TABLE t1 DROP PRIMARY KEY;
 DROP TABLE t1;
+
+# BUG#3899
+create table t1 (a int, b int, key(a));
+insert into t1 values (1,1), (2,2);
+--error 1091
+alter table t1 drop key no_such_key;
+alter table t1 drop key a;
+drop table t1;

sql/sql_yacc.yy

@@ -1817,7 +1817,7 @@ alter:
 	  lex->create_info.db_type= DB_TYPE_DEFAULT;
 	  lex->create_info.default_table_charset= thd->variables.collation_database;
 	  lex->create_info.row_type= ROW_TYPE_NOT_USED;
-	  lex->alter_info.clear();          
+	  lex->alter_info.reset();          
 	  lex->alter_info.is_simple= 1;
 	  lex->alter_info.flags= 0;
 	}