dbus-send: Reassure the compiler that secondary_type is initialized

dbus-send: Reassure the compiler that secondary_type is initialized

It's initialized to a non-trivial value whenever container_type
is DBUS_TYPE_DICT_ENTRY, and subsequently only used if
container_type is DBUS_TYPE_DICT_ENTRY, but Debian's gcc 7.2.0-7
doesn't seem to be able to infer that any more, causing build failure
under -Werror=maybe-uninitialized.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=102979
Reviewed-by: Philip Withnall <withnall@endlessm.com>