Start fuzzing HCI events + minor fix in hci_layer

Check event for validity before trying to send it,
to avoid triggering asserts inside HCI layer.

Rudimentary filtering right now since HCI reset is the only
thing that gets sent to the HAL.

Also, remove unhandled event assert, as controllers may send us
unsolicited events that (while unhandled) should not cause us
to crash.

Test: fuzz/run --host bluetooth_gd_hci_fuzz_test
Change-Id: Ic3181c0865b7519b89d2f03caf228c0010256a4a