BUG: memory access bug in read_csv causing segfault
closes https://github.com/pydata/pandas/issues/5664

the added test covers the original issue, though I cannot reproduce that on master; [the one mentioned in the comments](https://github.com/pydata/pandas/issues/5664#issuecomment-49332533) still segfaults on master (if repeated few times) and is fixed by this pr.
