[mc] Fix bug when parsing /proc/self/maps

[mc] Fix bug when parsing /proc/self/maps

Sometimes, the heap is not found by the MC :

* the MC expects the heap to be anonymous/unlabelled in
  /proc/self/maps;

* the heap may be labelled as as [stack:$tid] in /proc/self/maps at
  some instants of the execution of the program because the tasks
  stacks are allocated on the heap.