The `git_pqueue` struct allows being fixed in its total number of
entries. In this case, we simply throw away items that are
inserted into the priority queue by examining wether the new item
to be inserted has a higher priority than the previous smallest
one.

This feature somewhat contradicts our pqueue implementation in
that it is allowed to not have a comparison function. In fact, we
also fail to check if the comparison function is actually set in
the case where we add a new item into a fully filled fixed-size
pqueue.

As we cannot determine which item is the smallest item in absence
of a comparison function, we fix the `NULL` pointer dereference
by simply dropping all new items which are about to be inserted
into a full fixed-size pqueue.