avformat/http: return ENOSYS instead of UINT64_MAX for unknown filesize

This matches the behavior of e.g. the pipe: protocol, which returns ENOSYS
on account of ffurl_seek() not being implemented.

The previous behavior of returning s->filesize directly is almost surely a
bug, as s->filesize is UINT64_MAX when never initialized.

Sponsored-by: nxtedition AB
Signed-off-by: Niklas Haas <git@haasn.dev>

libavformat/http.c

@@ -2096,7 +2096,7 @@ static int64_t http_seek_internal(URLContext *h, int64_t off, int whence, int fo
     uint8_t discard[4096];
 
     if (whence == AVSEEK_SIZE)
-        return s->filesize;
+        return s->filesize == UINT64_MAX ? AVERROR(ENOSYS) : s->filesize;
     else if ((s->filesize == UINT64_MAX && whence == SEEK_END))
         return AVERROR(ENOSYS);