net: http_client: Fix handling of POLLHUP

POLLHUP event may be returned even if there is still data to read
and POLLIN is set. To ensure all data is consumed, check for
POLLHUP after handling POLLIN.

https://man7.org/linux/man-pages/man2/poll.2.html



Signed-off-by: Juha Ylinen <juha.ylinen@nordicsemi.no>