Add basic null checks for InternalNode head when in use.

Fixes issue #63.

I've added a check in the cue constructor for whether or not InternalNode is
valid. If it's not then it assigns it a safe value of NULL.  Likewise in the
InternalNode constructor it checks if the webvtt_node is valid before it tries
to access its data.

Please comment @caitp or @rillian.