Don't allocate memory for 0-page-sized requests

Previously the allocator would return all availble memory for a
request of 0 pages. This is rather counter-intuitive and left no
memory for subsequent allocations.

kernel/mem.c

@@ -553,6 +553,9 @@ static void *mckernel_allocate_aligned_pages_node(int npages, int p2align,
 #endif
 	int numa_id;
 
+	if(npages <= 0)
+		return NULL;
+
 	/* Not yet initialized or idle process */
 	if (!cpu_local_var_initialized ||
 			!cpu_local_var(current) ||