pipetransport: Fix a Coverity false-positive
pipetransport: Fix a Coverity false-positive

Coverity is flagging a false positive because it doesn't understand that
it's not possible for g_bytes_new_from_bytes() to return NULL (and
therefore, not possible for @channel to be set if @payload is %NULL
after a call to cockpit_transport_parse_frame().  This gives us a chance
to do some more consistent memory handling (read: introduce g_autoptr)
in cockpit_transport_read_from_pipe().

mvollmer/cockpit,cockpit-project/cockpit,garrett/cockpit,mvollmer/cockpit,garrett/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,garrett/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,garrett/cockpit,martinpitt/cockpit,garrett/cockpit