Fix bug #55138 PDO OCI cannot insert more than 1332 bytes

According to Oracle docs `value_sz` defines size in bytes, not characters.
`OCIBindByName()` docs:
http://docs.oracle.com/cd/E16655_01/appdev.121/e17625/oci16rel003.htm#LNOCI17140
```
value_sz (IN)
The maximum size possible in bytes of any data value (passed using valuep) for this bind variable. 
This size is always expected to be the size in bytes. 
```

This fix not covers VARCHAR2 extended mode in new Oracle versions.
See `max_string_size = extended` at
http://docs.oracle.com/cd/E16655_01/appdev.121/e17625/oci03typ.htm#LNOCI16271