lavf/rtmpproto: Don't unref uninitialized buffers

This happens if ffurl_open_whitelist fails and stream is unset.
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

libavformat/rtmpproto.c

@@ -1111,7 +1111,7 @@ static int rtmp_calc_swfhash(URLContext *s)
     RTMPContext *rt = s->priv_data;
     uint8_t *in_data = NULL, *out_data = NULL, *swfdata;
     int64_t in_size;
-    URLContext *stream;
+    URLContext *stream = NULL;
     char swfhash[32];
     int swfsize;
     int ret = 0;