stagefright: matroska: don't crash on parsing error

MatroskaExtractor crashed with an assertion if mkvparser::ParseNext()
returned a negative number.  Now handle all error values the same
way as the EOF.

Change-Id: I173c4f878d692a0cbdb915ad1118d0686249d625
Signed-off-by: Lajos Molnar <lajos@google.com>
Bug: 8433794