Fix bug in MediaBufferGroup new buffer allocation

MediaBufferGroup could allocate buffers of size 0, but this
was masked because we pre-allocated many more buffers. Now
that we allocate fewer buffers upfront, it's more likely
to hit this case.

Bug: 29125703
Test: CTS

Change-Id: Iba666f161be6d7c8bf09e1b48b1fd80750685a57