Add forgotten break

In the allocation tracker, not only did I use a dog-slow linear search,
I forgot to stop on the first empty spot, and actually used the last
empty spot... Add the missing break, which made leak detection 10% faster.

A better implementation would be preferable, but this is a low hanging
fruit

core/alloctracker.cc

@@ -23,6 +23,7 @@ void alloc_tracker::remember(void *addr, int size)
         if(!allocations[i].addr){
             // found a free spot, reuse it
             a = &allocations[i];
+            break;
          }
     }
     if (!a) {