[http] Treat any unexpected connection close as an error

[http] Treat any unexpected connection close as an error

iPXE currently checks that the server has not closed the connection
mid-stream (i.e. in the middle of a chunked transfer, or before the
specified Content-Length has been received), but does not check that
the server got as far as starting to send data.  Consequently, if the
server closes the connection before any data is transferred (e.g. if
the server gives up waiting while iPXE performs the validation steps
for TLS), then iPXE will treat this as a successful transfer of a
zero-length file.

Fix by checking the RX connection state, and forcing an error if the
server has closed the connection at an unexpected point.

Originally-fixed-by: Marin Hannache <mareo@mareo.fr>
Signed-off-by: Michael Brown <mcb30@ipxe.org>