lavf: simplify format_child_class_next()

And fix the error introduced when adding private option to avio.
See 32caa7b13c

libavformat/options.c

@@ -53,26 +53,29 @@ static const AVClass *format_child_class_next(const AVClass *prev)
     AVInputFormat  *ifmt = NULL;
     AVOutputFormat *ofmt = NULL;
 
-    while (prev && (ifmt = av_iformat_next(ifmt)))
+    if (!prev)
+#if !FF_API_OLD_AVIO
+        return &ffio_url_class;
+#else
+    prev = (void *)&ifmt; // Dummy pointer;
+#endif
+
+    while ((ifmt = av_iformat_next(ifmt)))
         if (ifmt->priv_class == prev)
             break;
-    if ((prev && ifmt) || (!prev))
+
+    if (!ifmt)
+        while ((ofmt = av_oformat_next(ofmt)))
+            if (ofmt->priv_class == prev)
+                break;
+    if (!ofmt)
         while (ifmt = av_iformat_next(ifmt))
             if (ifmt->priv_class)
                 return ifmt->priv_class;
 
-    while (prev && (ofmt = av_oformat_next(ofmt)))
-        if (ofmt->priv_class == prev)
-            break;
-    if ((prev && ofmt) || (!prev))
-        while (ofmt = av_oformat_next(ofmt))
-            if (ofmt->priv_class)
-                return ofmt->priv_class;
-
-#if !FF_API_OLD_AVIO
-    if (prev != &ffio_url_class)
-        return &ffio_url_class;
-#endif
+    while (ofmt = av_oformat_next(ofmt))
+        if (ofmt->priv_class)
+            return ofmt->priv_class;
 
     return NULL;
 }