Merge from Widevine repo of http://go/wvgerrit/40883
The oemcrypto dynamic adapter will try to load a keybox from the
filesystem if oemcrypto reports it doesn't have one. Previously, it
would check the size of the file, and then open the file before
responding to errors. Opening a non-existant file would result in a
segfault.
With this CL, we do not try to open the file if the size is not
positive.
Test: unit tests
Bug: 72129404
Change-Id: I243b5d49837455c0ce07802e32e4d7e809286cfd
4c7e6ca78b