fix bug in zstr s_send_string

My buildbot instance indicated that there was a fault in pyczmq, however upon further investigation it turned out to be in czmq. zmq_sendmsg returns the number of bytes sent if successful. Due to the recent code change delivered by #719 a successful send is now falling into the error handling block.

The pyczmq was detecting this error because it is explicitly checking the return code from a zstr_send. Perhaps we need to think about beefing up the error checking in the czmq zstr_test function so as to detect these regressions sooner?