say: fix NULL pointer dereference in log_syslog_init

If opts.identity is NULL and strdup is failed we do NULL pointer
dereference when reporting the error. Let's just panic if strdup() failed.
While at it replace another strdup() with xstrdup() in this function. Our
current approach is to panic on runtime OOM.

Closes tarantool/security#128

NO_TEST=issue is not possible after the fix
NO_CHANGELOG=not reproducible
NO_DOC=bugfix

(cherry picked from commit 47b72f44986797466b95b9431a381dbef7dd64fd)