Fix benign unsigned overflow in OggExtractor

When computing mCurrentPageSamples it was possible to have a harmless
unsigned integer overflow during the conf pages leading to false
positives with fsanitize integer. To prevent the false positives clamp
the result to 0.

Bug: 23488745
Bug: 23110888
Change-Id: I0769cb4a915d45b00ea43f2abbefe9ee46165cc7