libvpx

Properly validate data size

With "show_existing_frame" frames:
Minimum data size for profile 0 and 1 is 1 byte (8bits)
Minimum data size for profile 2 and 3 is 2 bytes (9bits)

Otherwise:
Minimum data size is 8 bytes.

This resolves the VP9 failure in fuzzing test build #56.

Change-Id: I146d9d37688f535dd68d24aacc76d464ccffdf04