mon/MonMap.h: fix potential segfault, add assert()

mon/MonMap.h: fix potential segfault, add assert()

Fix for:

CID 1297860 (#1 of 1): Argument cannot be negative (NEGATIVE_RETURNS)
1. Condition this->mon_addr.count(n), taking true branch
2. negative_return_fn: Function this->get_rank(n) returns a
   negative number. [show details]
3. var_assign: Assigning: signed variable m = get_rank.
4. negative_returns: m is passed to a parameter that cannot be
   negative.

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