tests: unit: add NULL-pointer check

In CID 1432593, Coverity complains that the result of qdict_crumple()
might leak if it is not a dictionary.  This is not a practical concern
since the test would fail immediately with a NULL pointer dereference
in qdict_size().

However, it is not nice to depend on qdict_size() crashing, so add an
explicit assertion that that the crumpled object was indeed a dictionary.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tests/unit/check-block-qdict.c

@@ -504,7 +504,7 @@ static void qdict_crumple_test_empty(void)
     src = qdict_new();
 
     dst = qobject_to(QDict, qdict_crumple(src, &error_abort));
-
+    g_assert(dst);
     g_assert_cmpint(qdict_size(dst), ==, 0);
 
     qobject_unref(src);