lwip: Fixed invalid DEBUGF level SERIOUS when pbuf_header fails (it returns '1': this is not serious!)
lwip: Fixed invalid DEBUGF level SERIOUS when pbuf_header fails (it returns '1': this is not serious!)

Upstream commit: 631c458c55aae868a286446a9d42951ef5e1e126

johnkingsley/aprinter,ambrop72/aprinter,ambrop72/aprinter,ambrop72/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,ambrop72/aprinter,johnkingsley/aprinter,ambrop72/aprinter,johnkingsley/aprinter,ambrop72/aprinter,johnkingsley/aprinter,ambrop72/aprinter