Check SetMap request length carefully.

Avoid out of bounds memory accesses on too short request.

ZDI-CAN 11572 /  CVE-2020-14360

This vulnerability was discovered by:
Jan-Niklas Sohn working with Trend Micro Zero Day Initiative

Signed-off-by: Matthieu Herrb <matthieu@herrb.eu>
(cherry picked from commit 446ff2d3)
3 jobs for backport-562 in 3 minutes and 38 seconds (queued for 2 seconds)