A previous CL broke the ASAN poisoning calculation in mallocgc by not
taking into account a possible allocation header, so the beginning of
the following allocation could have been poisoned.
This mostly isn't a problem, actually, since the following slot would
usually just have an allocation header in it that programs shouldn't be
touching anyway, but if we're going a word-past-the-end at the end of a
span, we could be poisoning a valid heap allocation.
Change-Id: I76a4f59bcef01af513a1640c4c212c0eb6be85b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/622295
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Keith Randall <khr@google.com>