(This is a merge of http://go/wvgerrit/140850.)
This patch fixes a number of minor issues in the codebase (mostly
instances of 0-as-nullptr, but also some member shadowing and a missing
override) that were being hidden by the fact that depending on Protobuf
disables these diagnostics. And which will be unhidden when a later
patch removes that behavior from Protobuf.
Bug: 208304830
Test: x86-64
Change-Id: I4b0b1264748880b3726a6388d589868d898f949e
df23c7da03