libbinder_ndk: fix failure when dump/shell are unset

People directly using libbinder_ndk functions who didn't create a debug
dump function function would fail to initialize that pointer, and
potentially crash. Those who didn't create a shell function were
guaranteed to crash. This wasn't noticed because the C++ wrappers which
are the recommended way to use libbinder_ndk always set these functions.

Bug: 161812320
Test: unit tests

Merged-In: I1f6909531bc640097f3f48c4a558fd03f2fa62cb
Change-Id: I1f6909531bc640097f3f48c4a558fd03f2fa62cb
(cherry picked from commit deb53467)