Bug #59197 double quote in field comment prevents foreign key constraint creation

Description:
A double quote (") in a field comment prevents foreign key constraints from being created properly. Instead they are created as simple keys. Similar to http://bugs.mysql.com/bug.php?id=39793

How to repeat:
-- With no double quote, we get the expected behavior (the error is expected because the referenced table doesn't exist):
mysql> CREATE TABLE t (a int COMMENT '', FOREIGN KEY (a) REFERENCES doesNotExist (a)) ENGINE=InnoDB;
ERROR 1005 (HY000): Can't create table 'test.t' (errno: 150)

-- ought to fail just the same as the above, but doesn't:
mysql> CREATE TABLE t (a int COMMENT '"', FOREIGN KEY (a) REFERENCES doesNotExist (a)) ENGINE=InnoDB;
Query OK, 0 rows affected (0.02 sec)

mysql> SHOW CREATE TABLE t;
+-------+-------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                      |
+-------+-------------------------------------------------------------------------------------------------------------------+
| t     | CREATE TABLE `t` (
  `a` int(11) DEFAULT NULL COMMENT '"',
  KEY `a` (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+-------+-------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

While dict_strip_comments() handles "\'" as quote, function dict_scan_to() only recognize  `` or "". So in dict_create_foreign_constraints_low(), following statement would not find FOREIGN keyword, as it recognize the single " character and consider everything after it are in quote, as it will not able to find the ending " character:

#1  0x085dbb81 in dict_create_foreign_constraints (trx=0x90a9ea8, 
    sql_string=0x90b9438 "CREATE TABLE t (a int COMMENT '\"', FOREIGN KEY (a) REFERENCES doesNotExist (a)) ENGINE=InnoDB", sql_length=93, 
    name=0xb2c6458c "test/t", reject_fks=0)

dict_create_foreign_constraints_low() -> ptr2 = dict_scan_to(ptr, "FOREIGN");

Fix would be make dict_scan_to() handle "\'" as quote.

From 5.1 plugin, the table structure is allocated with mem_heap_zalloc(), so it is not necessary explicitly set the variable to zero in dict_mem_table_create() any more.

Fix checked in 5.1 built-in, plugin.

Pushed into mysql-trunk 5.6.2 (revid:vasil.dimov@oracle.com-20110108150732-8qygun7nuaqf9d3u) (version source revid:vasil.dimov@oracle.com-20110108150646-44c9j7ck64ocey31) (merge vers: 5.6.2) (pib:24)

Pushed into mysql-5.5 5.5.9 (revid:vasil.dimov@oracle.com-20110108150508-gpanhz48z8069qot) (version source revid:vasil.dimov@oracle.com-20110108150048-b1y9m8xe72hay0ch) (merge vers: 5.5.9) (pib:24)

Pushed into mysql-5.1 5.1.55 (revid:vasil.dimov@oracle.com-20110108145923-0tjqdpa5w3d6tuwn) (version source revid:vasil.dimov@oracle.com-20110108145923-0tjqdpa5w3d6tuwn) (merge vers: 5.1.55) (pib:24)

Added to changelog:

The presence of a double quotation mark inside the COMMENT field for
a column could prevent a foreign key constraint from being created
properly.

Changelog entry appears in 5.6.2, 5.5.9, 5.1.55.