pqueue: resolve possible NULL pointer dereference

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.

src/pqueue.c

@@ -86,8 +86,9 @@ int git_pqueue_insert(git_pqueue *pq, void *item)
 	if ((pq->flags & GIT_PQUEUE_FIXED_SIZE) != 0 &&
 		pq->length >= pq->_alloc_size)
 	{
-		/* skip this item if below min item in heap */
-		if (pq->_cmp(item, git_vector_get(pq, 0)) <= 0)
+		/* skip this item if below min item in heap or if
+		 * we do not have a comparison function */
+		if (!pq->_cmp || pq->_cmp(item, git_vector_get(pq, 0)) <= 0)
 			return 0;
 		/* otherwise remove the min item before inserting new */
 		(void)git_pqueue_pop(pq);

tests/core/pqueue.c

@@ -93,7 +93,29 @@ void test_core_pqueue__max_heap_size(void)
 	cl_assert_equal_i(0, git_pqueue_size(&pq));
 
 	git_pqueue_free(&pq);
+}
+
+void test_core_pqueue__max_heap_size_without_comparison(void)
+{
+	git_pqueue pq;
+	int i, vals[100] = { 0 };
+
+	cl_git_pass(git_pqueue_init(&pq, GIT_PQUEUE_FIXED_SIZE, 50, NULL));
+
+	for (i = 0; i < 100; ++i)
+		cl_git_pass(git_pqueue_insert(&pq, &vals[i]));
 
+	cl_assert_equal_i(50, git_pqueue_size(&pq));
+
+	/* As we have no comparison function, we cannot make any
+	 * actual assumptions about which entries are part of the
+	 * pqueue */
+	for (i = 0; i < 50; ++i)
+		cl_assert(git_pqueue_pop(&pq));
+
+	cl_assert_equal_i(0, git_pqueue_size(&pq));
+
+	git_pqueue_free(&pq);
 }
 
 static int cmp_ints_like_commit_time(const void *a, const void *b)