bug#19563: grep -F: fix a heap buffer (read) overrun

[Jim Meyering]

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.

0001-grep-avoid-false-positive-UMR.patch
Description: Binary data

0002-grep-F-fix-a-heap-buffer-read-overrun.patch
Description: Binary data

0003-tests-add-support-for-ASAN-memory-poisoning.patch
Description: Binary data