another c2s crash

[tofu]

This seems to be sx related.

Core was generated by `/home/chesspark/production/bin/c2s -c /home/chesspark/production/etc/jabberd/c2'.
Program terminated with signal 6, Aborted.
#0  0xffffe410 in __kernel_vsyscall ()
(gdb) backtrace
#0  0xffffe410 in __kernel_vsyscall ()
#1  0xf7b7b811 in raise () from /lib/tls/i686/cmov/libc.so.6
#2  0xf7b7cfb9 in abort () from /lib/tls/i686/cmov/libc.so.6
#3  0xf7bb0d3a in __fsetlocking () from /lib/tls/i686/cmov/libc.so.6
#4  0xf7bb84dc in mallopt () from /lib/tls/i686/cmov/libc.so.6
#5  0xf7bb8672 in free () from /lib/tls/i686/cmov/libc.so.6
#6  0xf7c9e566 in XML_ParserFree () from /usr/lib/libexpat.so.1
#7  0x08059568 in sx_free (s=0x889c658) at sx.c:97
#8  0x080532fb in main (argc=Cannot access memory at address 0x201c
) at main.c:710

[tofu]

Another assert core this is for the newest version.

Core was generated by `/home/chesspark/production/bin/c2s -c /home/chesspark/production/etc/jabberd/c2'.
Program terminated with signal 6, Aborted.
#0  0xffffe410 in __kernel_vsyscall ()
(gdb) bt
#0  0xffffe410 in __kernel_vsyscall ()
#1  0xf7b17811 in raise () from /lib/tls/i686/cmov/libc.so.6
#2  0xf7b18fb9 in abort () from /lib/tls/i686/cmov/libc.so.6
#3  0xf7b10fbf in __assert_fail () from /lib/tls/i686/cmov/libc.so.6
#4  0x08056549 in sx_nad_write_elem (s=0x89bb390, nad=0x6, elem=0) at io.c:397
#5  0x08050496 in c2s_router_sx_callback (s=0x808aba8, e=event_PACKET, data=0x86aff80, arg=0x806f008) at c2s.c:1029
#6  0x08057ff5 in __sx_event (file=0x806a928 "io.c", line=134, s=0x808aba8, e=event_PACKET, data=0x86aff80) at sx.c:330
#7  0x08056b6a in _sx_process_read (s=0x808aba8, buf=0x8e3c2e8) at io.c:134
#8  0x08057036 in sx_can_read (s=0x808aba8) at io.c:218
#9  0x0804efc4 in c2s_router_mio_callback (m=0x808aec0, a=action_READ, fd=0x808b100, data=0x0, arg=0x806f008) at c2s.c:1202
#10 0x0805e481 in _mio_run (m=0x808aec0, timeout=5) at mio_impl.h:257
#11 0x08053bf5 in main (argc=Cannot access memory at address 0x45c8
) at main.c:688
(gdb) frame 4
#4  0x08056549 in sx_nad_write_elem (s=0x89bb390, nad=0x6, elem=0) at io.c:397
397         assert((int) (nad != NULL));

[smoku]

(In [476]) Fixes #138 - null nad after disconnect