heap_insert unsafe

Although understandably an edge case, heap_insert should fail to insert a new node if heap->nelts == UINT_MAX.

It is possible, though unlikely, that unsigned int is 16 bit. Even in a 32 bit world, hitting UINT_MAX becomes more likely as machines get faster.

At the very least it should assert(heap->nelts < UINT_MAX). It would be better to abort() than blindly corrupt the data structure.