main: fix crash by --tcmu-log-dir/-l
main: fix crash by --tcmu-log-dir/-l

Core was generated by `./tcmu-runner -l /var/log'.
Program terminated with signal 11, Segmentation fault.
 #0  0x00007f9edc0be8f1 in __strlen_sse2_pminub () from /lib64/libc.so.6
(gdb) where
 #0  0x00007f9edc0be8f1 in __strlen_sse2_pminub () at /lib64/libc.so.6
 #1  0x0000000000411924 in main (argc=3, argv=0x7ffed1977b78) at
 /data/tcmu-runner/main.c:943
(gdb) exit

Signed-off-by: Xiubo Li <67e5e1ee902b4e67bce18ea43d9c4b0430ce5e6c@cmss.chinamobile.com>

open-iscsi/tcmu-runner,agrover/tcmu-runner,open-iscsi/tcmu-runner