Accesses in the header (or trailing padding) of a malloc block should
not be associated with any mallocation since only the chunk-sized slots
actually get returned by malloc.
Basically, allow address-to-chunk lookup to fail, and handle such
failures gracefully at call sites.
Fixes#5706.
We don't want to audit accesses into the region *while* we're setting
up malloc tracking for it. Fetching the chunk size from the header
was tripping up the auditing code.
(...and ASSERT_NOT_REACHED => VERIFY_NOT_REACHED)
Since all of these checks are done in release builds as well,
let's rename them to VERIFY to prevent confusion, as everyone is
used to assertions being compiled out in release.
We can introduce a new ASSERT macro that is specifically for debug
checks, but I'm doing this wholesale conversion first since we've
accumulated thousands of these already, and it's not immediately
obvious which ones are suitable for ASSERT.
