Fix reading past array end in FAT16 driver

src/kernel/fat16.c

@@ -357,7 +357,7 @@ fs_node_t *fat16_open_at(const fs_node_t *directory, uint16_t index, uint64_t fl
     ei++;
   }
 
-  fs_node_t *result = open_entry(dir_cluster, entries, ei);
+  fs_node_t *result = ei < entries_count ? open_entry(dir_cluster, entries, ei) : NUL;
 
   memory_free(entries);