avformat/hls: ignore http_persistent for segments requring crypto

Encrypted HLS segments have regular http:// urls, but open_input()
actually prefixes them with crypto+ before calling open_url(), so
they end up using the crypto protocol and not the http protocol.

This means invoking ff_http_do_new_request will fail, so we avoid
calling it in the first place. After the earlier http.c commit,
the failure results in a warning printed to the user. In earlier
versions, the failure would cause a segfault.

Signed-off-by: Aman Gupta <aman@tmm1.net>