FileDescription: Disallow construction with a null File

It's not valid for a FileDescription to not have a file, so let's
disallow it by taking a File& (or FIFO&) in the constructor.

Kernel/FileSystem/FIFO.cpp

@@ -32,7 +32,7 @@ NonnullRefPtr<FIFO> FIFO::create(uid_t uid)
 
 NonnullRefPtr<FileDescription> FIFO::open_direction(FIFO::Direction direction)
 {
-    auto description = FileDescription::create(this);
+    auto description = FileDescription::create(*this);
     attach(direction);
     description->set_fifo_direction({}, direction);
     return description;