If a hdrlen was provided that was greather than the actual
message length, a negative attributes length would result.
This was typically happening for RTM_GETLINK requests where
we can get a away with a 4 bytes header on the request side
but the response would use a 16 bytes header.
This resulted in strange -8 bytes leftover debug messages.
Signed-off-by: Thomas Graf <tgraf@suug.ch>