Perform null check before free

[petesmc]

https://github.com/esheldon/fitsio/blob/90697250a609cee86a9b3c4d9567bd0fa1da8260/fitsio/fitsio_pywrap.c#L303-L315

Line 308, probably want to null check before attempting to free.

Also the free(slist->data); should be inside the conditional block.

[esheldon]

free does nothing if the pointer is NULL