error check vp9 superframe parsing

This patch insures that the last byte of a chunk that contains a
valid superframe marker byte,  actually has a proper superframe index.
If not it returns an error.

As part of doing that the file : vp90-2-15-fuzz-flicker.webm now fails
to decode properly and moves to the invalid file test from the test
vector suite.

Change-Id: I5f1da7eb37282ec0c6394df5c73251a2df9c1744