We already validated that the required headers for each type of D-Bus
message were present. However, we didn’t validate that they contained a
variant of the right type. This could lead to functions like
`g_dbus_message_get_path()` returning `NULL` unexpectedly.

This failure could only be hit when using GDBus in peer-to-peer mode, or
with a D-Bus server which didn’t validate the headers itself. The
reference D-Bus server does validate the headers, and doesn’t forward
invalid messages to clients.

Signed-off-by: Philip Withnall <philip@tecnocode.co.uk>

Fixes: #3061