net: lib: http_server: prevent busfault if socket family is AF_UNSPEC

If webserver uses custom socket creation,
and it's family is AF_UNSPEC, net_sprint_addr will return NULL,
which leads to LOG_DBG crashing with busfault.
Add NULL check for that.

Signed-off-by: Andrey Dodonov <Andrey.Dodonov@endress.com>