avformat/http: return EINVAL from ff_http_do_new_request() if re-used with different hostname

This will prevent improper use of ff_http_do_new_request() if the user
tries to send a request for a different host to a previously connected
persistent http/1.1 connection.

Signed-off-by: Aman Gupta <aman@tmm1.net>
Signed-off-by: Karthick J <kjeyapal@akamai.com>