Add a nil guard to prevent crashes when a link's media type cannot be parsed

[leonardr]

This branch fixes a problem I've seen in real EPUBs (sample available on request) which causes a crash on load.

The problem is triggered by a link in the OPF with a media type that causes mediatype.New to error; in my case it was "application/application/x-font-ttf". The Link object ends up with a nil MediaType. There are a number of places in the code that call functions on link.MediaType without a nil guard, which causes the crash..

In this branch I added nil guards everywhere in the code I think is relevant. The end result should be that a link with a nil media type is not anything in particular: not a bitmap, not HTML, etc.

An alternative implementation would be to define a sentinel "unknown" MediaType to stand in for the case where mediatype.New can't parse the actual media type. Then we can guarantee mediatype.New always returns a MediaType and the nil guards won't be necessary. That's a more systematic change, so before working on it, I want to understand your general policy on having go-toolkit handle invalid data in EPUBs.