gdbusmessage: Remove arbitrary restriction on minimum D-Bus blob length

The code in `g_dbus_message_new_from_blob()` has now been fixed to
correctly error out on all truncated messages, so there’s no need for an
arbitrary programmer error if the input is too short to contain a valid
D-Bus message header.

Signed-off-by: Philip Withnall <>

Helps: #2528
13 jobs for 2528-dbus-message-truncation in 19 minutes and 5 seconds (queued for 4 seconds)