avformat/http: require seekability only when seeking to position != 0

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/http.c

@@ -1427,7 +1427,7 @@ static int64_t http_seek_internal(URLContext *h, int64_t off, int whence, int fo
              ((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)
+    else if ((s->filesize == -1 && whence == SEEK_END))
         return AVERROR(ENOSYS);
 
     if (whence == SEEK_CUR)
@@ -1440,6 +1440,9 @@ static int64_t http_seek_internal(URLContext *h, int64_t off, int whence, int fo
         return AVERROR(EINVAL);
     s->off = off;
 
+    if (s->off && h->is_streamed)
+        return AVERROR(ENOSYS);
+
     /* we save the old context in case the seek fails */
     old_buf_size = s->buf_end - s->buf_ptr;
     memcpy(old_buf, s->buf_ptr, old_buf_size);