dbus_message_demarshal: Set error if we can't allocate the loader

dbus_message_demarshal: Set error if we can't allocate the loader

Backported from 1.13.0. Previously this was fixed in the dbus-1.14 branch
but unfixed in the dbus-1.12 branch, but we need it fixed in dbus-1.12
if we want the additional test coverage for dbus#413 to pass.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=100317
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit ecbcffae19ff1b811fc7d0d602458c0f00dd6771)