avformat_open_input(): Add braces to shut up gcc warning.

libavformat/utils.c:599: warning: missing braces around initializer
libavformat/utils.c:599: warning: (near initialization for ‘ap.time_base’)

libavformat/utils.c

@@ -597,7 +597,7 @@ int avformat_open_input(AVFormatContext **ps, const char *filename, AVInputForma
 {
     AVFormatContext *s = *ps;
     int ret = 0;
-    AVFormatParameters ap = { 0 };
+    AVFormatParameters ap = { { 0 } };
     AVDictionary *tmp = NULL;
 
     if (!s && !(s = avformat_alloc_context()))