receive_encrypted_request(): Properly deal with pending data

receive_encrypted_request(): Properly deal with pending data

... that has already been received and is thus invisible to
data_is_available().

Previously encrypted client requests that were too large
to be read with a single ssl_recv_data() call could be
rejected as invalid if all the data arrived quickly enough.

Apparently this happended frequently on gmail due to
large Cookies.

Reported by: Robert Klemme
Sponsored by: Robert Klemme