Fixed ODBC WString size handling.

This fixes a bug where Poco would return a string 1 character shorter than is actually in the DB (when the string in the DB uses the full length of a CHAR(N) or VARCHAR(N) column).

Test case:
MySQL 5.1 ODBC driver, database with table A, column B defined as CHAR(4), open session to the DB `session`.
```cpp
string b = "1234";
session << "INSERT INTO A (B) VALUES (?)", bind(b), now;
session << "SELECT B FROM A", into(b), now;
assert(b == "1234");  // Fails; Poco returns b == "123"
```