Fix processing of mixed ID and SPTPS data.

When making a connection with another peer, the ID message and the first
SPTPS data is sent using separate send() calls, but they can be merged
along the way to the other side. When receiving the ID message, we
accidentily discarded the rest of the received packet. Handle this
properly.

The problem was found to trigger much more often on Linux kernel 4.4 in
KVM.