Check for nil pointer in queries. Will set the Cassandra column to null.

Could be a fix for https://github.com/gocql/gocql/issues/172

My intention was to allow queries like

    c.Query("INSERT INTO my_table (username, firstname, lastname, sex) VALUES (?, ?, ?, ?);",
    'j.doe', 'j.', 'doe', nil).Exec()

This resulted in a panic before.