lib-http: http-client-request - Fix payload assertions in http_client_request_send_mo...

lib-http: http-client-request - Fix payload assertions in http_client_request_send_more().

When the request payload is finished, both req->payload_input and
req->payload_output could be NULL, so the assertions on those being not NULL
need to happen after the check for req->payload_finished.

This particularly causes problems with the
blocking http_client_request_send/finish_payload() API, which constantly
modifies req->payload_input and sets it to NULL to finish the output.

This caused a panic:

Panic: file http-client-request.c: line 1232 (http_client_request_send_more): assertion failed: (req->payload_input != NULL)