Use strlen() in webvtt_string_getline() if (signed)len is negative

Changed prototype and definition of webvtt_string_getline() to accept a signed
'len' parameter.

This parameter is tested, and if it is negative, it will determine the length
of a string using strlen().

This fixes a test failure on travis
(https://travis-ci.org/caitp/webvtt/jobs/5297197/#L684)