Fix segfaults in ExtraNewLinesAfterCue.

I've fixed the segfaul in FileStructure.ExtraNewLinesAfterCue.
Problem was that it was trying to parse a cue when it got to the
end of the buffer and there was just new lines after it. I've set it
up to now consume all white space after a cue or cue text has been
parsed. It's seemed to fix a lot of the other tests too.
Add param checks to skipwhite function.

I've added param checks to the new webvtt_skipwhite function. I've
also moved the respective webvtt_skipwhite functions to their
respective files so that the names do not conflict with one another.
Add check for *pos < len in webvtt_skipwhite in parser.c.

I've also removed the check for *pos > 0 as this is unecessary because
pos is an unsigned int.
Reverse *pos < len and iswhite checks in skipwhite func.

I've reversed the conditional statements of *pos < len and webvtt_iswhite
in parser.c's webvtt_skipwhite function in order to ensure that we aren't
trying to dereference on an index that doesn't exist.
Merge pull request #136 from RickEyre/issue-15-2

Fix segfaults in ExtraNewLinesAfterCue.