Added helper routine to determine if cue is to be skipped.

returns nonzero if the cue has an ID but no cuesettings (the only
situation where we can really have a cue-id and encounter a newline or
something).

Might be helpful for debugging eg `p cue_is_incomplete(cue)', might
also in the future make code a bit more readable.

webvtt_bool cue_is_incomplete( const webvtt_cue * )
Merge pull request #262 from caitp/cue-is-incomplete

Added helper routine to determine if cue is to be skipped.
Convert plformat segfaults into test failures.

This doesn't fix the logic errors in the parser, it just adds
guards to return early instead of segfaulting. This lets us
continue to the next unit test and get better coverage.