fibheap: fix bug in child element remove

When the minimal node is extracted, all the child nodes are moved to the
rootlist, no special care is needed when executing application in
console environment with no stepping, however in GUI application where
every single step is important and necessary to render, removing child
nodes need to be performed properly, otherwise an infinite recursion in
rendering may occur, where both child and its parent reference the same
node being the child. (As it was mentioned above, this is not a problem
                       when executed in cli, as the parent node will be
                       extracted without any interruption from rendering
                       subroutines)