Handle the "no local address" case in send_sptps_data().

Handle the "no local address" case in send_sptps_data().

If choose_local_address() is unable to find a local address (e.g.
because of old nodes that don't send their local address information),
then send_sptps_data() ends up using uninitialized variables for the
socket and address.

This regression was introduced in
415910897122da0073a862784d148802ca390020. The commit took care of
handling that case in send_udppacket() but was missing the same fix
for send_sptps_data().

This bug was found by the clang static analyzer tool:
http://clang-analyzer.llvm.org/