common/HeartbeatMap: fix uninitialized variable

Introduced by me in 132045ce085e8584a3e177af552ee7a5205b13d8.  Thank you,
valgrind!

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 00cfe1d3af286ffab7660933415684f18449720c)