libiptc: Eliminate garbage access

libiptc: Eliminate garbage access

When adding a rule, valgrind prints:

Syscall param socketcall.setsockopt(optval) points to uninitialised byte(s)
   at 0x4A8165A: setsockopt (in /lib64/libc.so.6)
   by 0x4857A48: iptc_commit (libiptc.c:2676)
   by 0x10E4BB: iptables_main (iptables-standalone.c:61)
   by 0x49A3349: (below main) (in /lib64/libc.so.6)
 Address 0x4b63788 is 40 bytes inside a block of size 1,448 alloc'd
   at 0x484659F: calloc (vg_replace_malloc.c:1328)
   by 0x4857654: iptc_commit (libiptc.c:2564)
   by 0x10E4BB: iptables_main (iptables-standalone.c:61)
   by 0x49A3349: (below main) (in /lib64/libc.so.6)

This is because repl->counters is not initialized upon allocation. Since
the field is an array, make use of calloc() which implicitly does the
initialization.

Fixes: e37c0dc100c51 ("Revert the recent addition of memset()'s to TC_COMMIT. One of them is bogus and the other one needs more investigation to why valgrind is complaining.")
Signed-off-by: Phil Sutter <phil@nwl.cc>