error code DBUS_VALIDITY_UNKNOWN_OOM_ERROR = -4 for
        out of memory errors when validating signitures

        * dbus/dbus-marshal-header.c: use DBUS_VALIDITY_UNKNOWN_OOM_ERROR
        in places where we previously used DBUS_VALID and a FALSE return
        value to indicate OOM

        * dbus/dbus-marshal-validate.c (_dbus_validate_signature_with_reason):
        Use a stack to track the number of elements inside containers.  The
        stack values are then used to validate that dict entries have only two
        elements within them.
        (validate_body_helper): check the reason for failure when validating
        varients

        * dbus/dbus-message.c (load_message): use
        DBUS_VALIDITY_UNKNOWN_OOM_ERROR in places where we previously used
        DBUS_VALID and a FALSE return value to indicate OOM

        * doc/TODO: remove "- validate dict entry number of fields" as this
        patch fixes it