Fix Q_ASSERT in direct2d qpa plugin

When calling end the refcount should be greater than zero, as calls to
begin/end should match.

Change-Id: I49adbf6f9f1c21cb60985ba4e02574e6c0841b74
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/plugins/platforms/direct2d/qwindowsdirect2ddevicecontext.cpp

@@ -80,7 +80,7 @@ public:
     bool end()
     {
         Q_ASSERT(deviceContext);
-        Q_ASSERT(refCount >= 0);
+        Q_ASSERT(refCount > 0);
 
         bool success = true;
         refCount--;