Don't assert while a callback is registered

If an assert fails while a callback is registered and some data
is delivered that attempts to invoke the callback, it'll cause a
segfault since the callback is allocated on a part of the stack
that's no longer valid.

Bug: 183362725
Change-Id: I3a1f8fbf15c24e344fde8e5b44bb93681ef47d38
Test: Run VTS