prlimit: fix compiler warning [-Wmaybe-uninitialized]

prlimit: fix compiler warning [-Wmaybe-uninitialized]

sys-utils/prlimit.c:467:16: warning: 'hard' may be used uninitialized in this function [-Wmaybe-uninitialized]
  lim->rlim_max = hard;
  ~~~~~~~~~~~~~~^~~~~~
sys-utils/prlimit.c:456:15: note: 'hard' was declared here
  rlim_t soft, hard;
               ^~~~
sys-utils/prlimit.c:466:16: warning: 'soft' may be used uninitialized in this function [-Wmaybe-uninitialized]
  lim->rlim_cur = soft;
  ~~~~~~~~~~~~~~^~~~~~
sys-utils/prlimit.c:456:9: note: 'soft' was declared here
  rlim_t soft, hard;
         ^~~~

References: https://github.com/karelzak/util-linux/issues/1406
Signed-off-by: Karel Zak <kzak@redhat.com>