Modify hidl_pointer default constructor

Initialize hidl_pointer using _pad to ensure when a 32-bit
process sends a null hidl_handle within a compound type to
a 64-bit process, uninitialized memory does not get copied
over to it.

Bug: 35279588
Test: hidl_test

Change-Id: I364f21765db2f4702d8908aedcb2a55f9f71f785
1 file changed