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

Change-Id: I1f6909531bc640097f3f48c4a558fd03f2fa62cb
diff --git a/libs/binder/ndk/ibinder_internal.h b/libs/binder/ndk/ibinder_internal.h
index 5779427..902fe79 100644
--- a/libs/binder/ndk/ibinder_internal.h
+++ b/libs/binder/ndk/ibinder_internal.h
@@ -110,13 +110,13 @@
     const ::android::String16& getInterfaceDescriptor() const { return mInterfaceDescriptor; }
 
     // required to be non-null, implemented for every class
-    const AIBinder_Class_onCreate onCreate;
-    const AIBinder_Class_onDestroy onDestroy;
-    const AIBinder_Class_onTransact onTransact;
+    const AIBinder_Class_onCreate onCreate = nullptr;
+    const AIBinder_Class_onDestroy onDestroy = nullptr;
+    const AIBinder_Class_onTransact onTransact = nullptr;
 
     // optional methods for a class
-    AIBinder_onDump onDump;
-    AIBinder_handleShellCommand handleShellCommand;
+    AIBinder_onDump onDump = nullptr;
+    AIBinder_handleShellCommand handleShellCommand = nullptr;
 
    private:
     // This must be a String16 since BBinder virtual getInterfaceDescriptor returns a reference to