Fix out of boundary memory read in fuzz test on vpxdec

Fix out of boundary memory read in fuzz test on vpxdec

This commit fixes frame header decoding for superframe index, to
prevent out of boundary memory read triggered by fuzz test
vector. It resolves a chromium security violation issue
crbug.com/376802.

The issue was introduced in the change:

Add VPXD_SET_DECRYPTOR support to the VP9 decoder.
cl-id I88f86c8ff9af34e0b6531028b691921b54c2fc48

where the buffer was read before validation check on index offset
applied.

A test vector is added accordingly.

Change-Id: I41c988e776bbdd1033312a668e03a3dbcf44ca99