tests: matroskamux avoid -Wmaybe-uninitialized

tests: matroskamux avoid -Wmaybe-uninitialized

False positive for the three variables but some warnings like:

   ../tests/check/elements/matroskamux.c:875:10:
    warning: 'chapters_offset' may be used uninitialized in this function [-Wmaybe-uninitialized]
   *index = chapters_offset;
   ~~~~~~~^~~~~~~~~~~~~~~~~

The above is false positive as there is a gboolean to check if it was
initialized or not (found_chapters_declaration).