g_dbus_message_new_from_blob goes into infinite loop for certain inputs

This is a clear DoS attack vector, so I'm reporting this in a confidential issue

This causes https://oss-fuzz.com/testcase-detail/5746957250723840.

After some debugging I found the reason to be that the decoder wants to read an array length, but its at the end of the buffer, which gets interpreted as length 0 and it will continue infinitely.

The following diff fixes the problem.