Fix issue 11594: Check if the monitor is null in _d_monitorenter

In case of

```
        synchronized (null)
        {
                // ...
        }
```

runtime will try to access __monitor field on a null,
without checking if it's a null and it will segfault,
leaving programmer clueless.

https://issues.dlang.org/show_bug.cgi?id=11594

src/rt/monitor_.d

@@ -57,6 +57,11 @@ extern (C) void _d_monitordelete(Object h, bool det)
 }
 
 extern (C) void _d_monitorenter(Object h)
+in
+{
+    assert(h !is null, "Synchronized object must not be null.");
+}
+body
 {
     auto m = cast(Monitor*) ensureMonitor(h);
     auto i = m.impl;