Colleagues Nima Aghdaii and Yuliy Pisetsky found and fixed a heap
buffer overrun in grep's kwset.c.  The underlying bug, already hard to
trigger, would most often result in a "mere" heap UMR (uninitialized
memory read), but Yuliy constructed inputs (see the new test) that
cause a buffer overrun.

I'm attaching two other related patches:
  - grep: avoid false-positive UMR
  - tests: add support for ASAN memory poisoning

I expect to push these by Monday.