libbinder_ndk: dead binder when associating dead binder

It may be surprising that this is null. Add diagnostic message to assist
development. If/when we change this behavior, it will probably require a
new libbinder_ndk to avoid changing behavior.

Bug: 155793159
Test: repro error and check message:
07-08 00:56:55.250  3414  3418 E binderNdkDroppedTest: associateClass:
Cannot associate class 'IBinderNdkDroppedTest' to dead binder.

Change-Id: If9d897df1676dce4be1bd79327087dad918a2a73