Add accessors for when you're sure a linked list is not empty

kernel/src/memory/Heap.cpp

@@ -178,7 +178,7 @@ Result<void*> kmalloc(usize size)
         heap.append(block);
     }
 
-    HeapBlock* block = heap.first().value();
+    HeapBlock* block = heap.expect_first();
     while (block)
     {
         // Trying to find a free block...
@@ -330,7 +330,7 @@ void dump_heap_usage()
     }
     usize alloc_total = 0;
     usize alloc_used = 0;
-    HeapBlock* block = heap.first().value();
+    HeapBlock* block = heap.expect_first();
     while (block)
     {
         if (is_block_free(block))

kernel/src/thread/Scheduler.cpp

@@ -98,7 +98,7 @@ namespace Scheduler
 
         do {
             auto maybe_next = g_threads.next(g_current);
-            if (maybe_next.has_error()) g_current = g_threads.first().value();
+            if (maybe_next.has_error()) g_current = g_threads.expect_first();
             else
                 g_current = maybe_next.value();
 

luna/include/luna/LinkedList.h

@@ -106,11 +106,21 @@ template <typename T> class DoublyLinkedList
         return nonnull_or_error((T*)m_start_node);
     }
 
+    T* expect_first()
+    {
+        return first().value();
+    }
+
     Result<T*> last()
     {
         return nonnull_or_error((T*)m_end_node);
     }
 
+    T* expect_last()
+    {
+        return last().value();
+    }
+
     Result<T*> next(T* item)
     {
         return nonnull_or_error((T*)extract_node(item)->get_next());