Fix format truncation

[michaelortmann]

Found by: michaelortmann
Patch by: michaelortmann
Fixes:

One-line summary:
Fix format truncation

Additional description (if needed):
Found with gcc -Wformat-truncation=2:

gcc -std=gnu99 -Wformat-truncation=2 -pipe -Wall -I.. -I..  -DHAVE_CONFIG_H -I/usr/include  -c dcc.c
dcc.c: In function ‘dcc_bot_check_digest’:
dcc.c:613:39: warning: ‘@’ directive output may be truncated writing 1 byte into a region of size between 0 and 30 -Wformat-truncation=]
  613 |   snprintf(digest_string, 33, "<%lx%lx@", (long) getpid(),
      |                                       ^
dcc.c:613:3: note: ‘snprintf’ output between 5 and 35 bytes into a destination of size 33
  613 |   snprintf(digest_string, 33, "<%lx%lx@", (long) getpid(),
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  614 |            (unsigned long) dcc[idx].timeval);
      |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Test cases demonstrating functionality (if applicable):