prevent null dereference (#13)

serialization.nim

@@ -89,7 +89,10 @@ template loadFile*(Format: distinct type,
                    params: varargs[untyped]): auto =
   mixin init, ReaderType
   var stream = openFile(filename)
-  defer: stream[].close() # TODO: destructors
+  defer:
+    # TODO: destructors
+    if not stream.isNil:
+      stream[].close()
 
   # TODO:
   # Remove this when statement once the following bug is fixed: