OSDMonitor.cc: fix potential division by zero

Fix for:

CID 1232602 (#1 of 1): Division or modulo by zero (DIVIDE_BY_ZERO)
 divide_by_zero: In expression num_pg_copies / num_osds, division
 by expression num_osds which may be zero has undefined behavior.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>