Fix incorrect SPTPS session labels.

Fix incorrect SPTPS session labels.

To prevent a MITM from being able to splice different SPTPS sessions
together, each session has a unique label. snprintf() was used to fill a
buffer with the label, however the buffer was not big enough for the whole
label. Linux made sure the last byte of the buffer was a NUL character,
Windows did not, resulting in the two unable to connect to each other.