ANDROID: trusty: avoid using BUG_ON()

checkpatch warns about BUG_ON() because it is discouraged these days.
Instead, whenever possible the problem should be recovered from instead
(e.g. by returning an error code).

Change-Id: I21aff5e0349666c5de6ec003f1979b30d6ca0a75
Signed-off-by: Eric Biggers <ebiggers@google.com>