PTS.__cinit__: Prevent use of uninitialized security object

Previously, PTS.__cinit__ would leave sc uninitialized when passed an
invalid negative sec argument.

Fixes
afs/_pts.c: In function ‘__pyx_pf_3afs_4_pts_3PTS___cinit__’:
afs/_pts.c:2999:56: warning: ‘__pyx_v_sc’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     (__pyx_v_serverconns[__pyx_v_i]) = rx_NewConnection((__pyx_v_info.hostAddr[__pyx_v_i]).sin_addr.s_addr, (__pyx_v_info.hostAddr[__pyx_v_i]).sin_port, PRSRV, __pyx_v_sc, __pyx_t_9);
                                                        ^

Signed-off-by: Anders Kaseorg <andersk@mit.edu>