libstagefright: Fix crash in AVCDecoder during thumbnail creation.

-AVCDecoder reads the sequence parameter set values from the
 sequence parameter set id zero. The clip has valid sequence
 parameter set data at sequence parameter set id one.
-Invalid data at id zero causes NULL dereference and crash.
-Add fix to return error if sequence parameter set data at id
 zero points to invalid data.

Change-Id: If9b4507242f7ea0127f75a6957d8ad73b91ce6fa
CRs-Fixed: 284889