lavf: fix null pointer dereference in rdt

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -607,6 +607,9 @@ static int init_input(AVFormatContext *s, const char *filename)
     int ret;
     AVProbeData pd = {filename, NULL, 0};
 
+    if(s->iformat && !strlen(filename))
+        return 0;
+
     if (s->pb) {
         s->flags |= AVFMT_FLAG_CUSTOM_IO;
         if (!s->iformat)