avformat/libsrt: use a larger buffer for find_info_tag

The upper limit of strlen(streamid) is 512. Use a larger buffer for
future proof, for example, deal with percent-encoding.
Reviewed-by: Zhao Jun <barryjzhao@tencent.com>
Signed-off-by: Steven Liu <liuqi05@kuaishou.com>

libavformat/libsrt.c

@@ -529,7 +529,7 @@ static int libsrt_open(URLContext *h, const char *uri, int flags)
 {
     SRTContext *s = h->priv_data;
     const char * p;
-    char buf[256];
+    char buf[1024];
     int ret = 0;
 
     if (srt_startup() < 0) {