atrun: Do not assume that MAXLOGNAME <= 100.

The reserved space for fmt was exactly sufficient for a two-digit value of
MAXLOGNAME - 1.

PR:		bin/171815
Submitted by:	Jeremy Huddleston Sequoia
MFC after:	1 week

libexec/atrun/atrun.c

@@ -123,7 +123,7 @@ run_file(const char *filename, uid_t uid, gid_t gid)
     pid_t pid;
     int fd_out, fd_in;
     int queue;
-    char mailbuf[MAXLOGNAME], fmt[49];
+    char mailbuf[MAXLOGNAME], fmt[64];
     char *mailname = NULL;
     FILE *stream;
     int send_mail = 0;