block-qdict: Pacify Coverity after commit f1b34a248e9

Commit f1b34a248e9 replaced less-than-obvious test in
qdict_flatten_qdict() by the obvious one.  Sadly, it made something
else non-obvious: the fact that @new_key passed to qdict_put_obj()
can't be null, because that depends on the function's precondition
(target == qdict) == !prefix.

Tweak the function some more to help Coverity and human readers alike.

Fixes: CID 1393620
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/qobject/block-qdict.c b/qobject/block-qdict.c
index 36129e7..80c6530 100644
--- a/qobject/block-qdict.c
+++ b/qobject/block-qdict.c
@@ -97,7 +97,7 @@
     const QDictEntry *entry, *next;
     QDict *dict_val;
     QList *list_val;
-    char *new_key;
+    char *key, *new_key;
 
     entry = qdict_first(qdict);
 
@@ -106,10 +106,12 @@
         value = qdict_entry_value(entry);
         dict_val = qobject_to(QDict, value);
         list_val = qobject_to(QList, value);
-        new_key = NULL;
 
         if (prefix) {
-            new_key = g_strdup_printf("%s.%s", prefix, entry->key);
+            key = new_key = g_strdup_printf("%s.%s", prefix, entry->key);
+        } else {
+            key = entry->key;
+            new_key = NULL;
         }
 
         /*
@@ -125,19 +127,17 @@
          * well advised not to modify them altogether.)
          */
         if (dict_val && qdict_size(dict_val)) {
-            qdict_flatten_qdict(dict_val, target,
-                                new_key ? new_key : entry->key);
+            qdict_flatten_qdict(dict_val, target, key);
             if (target == qdict) {
                 qdict_del(qdict, entry->key);
             }
         } else if (list_val && !qlist_empty(list_val)) {
-            qdict_flatten_qlist(list_val, target,
-                                new_key ? new_key : entry->key);
+            qdict_flatten_qlist(list_val, target, key);
             if (target == qdict) {
                 qdict_del(qdict, entry->key);
             }
         } else if (target != qdict) {
-            qdict_put_obj(target, new_key, qobject_ref(value));
+            qdict_put_obj(target, key, qobject_ref(value));
         }
 
         g_free(new_key);