Reset LogTag to NULL in closelog(3). This fixes mysterious crashes

caused by dynamic PAM modules that call openlog(3) and closelog(3),
e.g. ports/security/pam_pwdfile.

What happened here is that the module first registered its "ident"
with openlog(3), then PAM library unloaded module with dlclose(3),
and the next call to syslog(3) resulted in SIGSEGV.

MFC after:	3 days

lib/libc/gen/syslog.c

@@ -335,6 +335,7 @@ closelog()
 {
 	(void)_close(LogFile);
 	LogFile = -1;
+	LogTag = NULL;
 	connected = 0;
 }