Fix compile warning -Wmaybe-uninitialized

[shinyaaa]

Compile error has occuered, so I fixed it.

In file included from /home/shinya/pgsql/17/include/server/c.h:1370,
                 from /home/shinya/pgsql/17/include/server/postgres_fe.h:25,
                 from pg_rman.h:13,
                 from init.c:10:
init.c: In function ‘do_init’:
/home/shinya/pgsql/17/include/server/port.h:242:33: warning: ‘sysid’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  242 | #define fprintf                 pg_fprintf
      |                                 ^~~~~~~~~~
init.c:43:21: note: ‘sysid’ was declared here
   43 |         uint64      sysid;
      |                     ^~~~~