(Merge from http://go/wvgerrit/29180)
initDataResemblesPSSH was not checking if the buffer was large enough to
contain the PSSH markers that it was looking for. Consequently, it could
read data past the end of the buffer.
Bug: 63076692
Test: Unit Tests
Test: Google Play Movies
Change-Id: I6a4a3fa4b05d98554645f3c3149569702c96fd66