http: Always allow no-op seek

This also allows checking stream position as per ffurl_seek() doxy.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavformat/http.c

@@ -901,6 +901,9 @@ static int64_t http_seek(URLContext *h, int64_t off, int whence)
 
     if (whence == AVSEEK_SIZE)
         return s->filesize;
+    else if ((whence == SEEK_CUR && off == 0) ||
+             (whence == SEEK_SET && off == s->off))
+        return s->off;
     else if ((s->filesize == -1 && whence == SEEK_END) || h->is_streamed)
         return AVERROR(ENOSYS);