Backtrace:

...
(gdb) bt
#0  die () at strace.c:313
#1  0x000000000040d883 in error_msg_and_die (fmt=fmt@entry=0x4640e8 "%s: got unexpected return value %d for snprintf(buf, %zu, %s)") at error_prints.c:74
#2  0x000000000042152a in xsnprintf_ (str=0x6f9a20 <res.6552> "if_nametoindex(\"\\x62\\x6c\\x69\\x6e\\x6", size=36, func=0x484ede <__func__.6555> "get_ifname", argstr=0x484eb0 "(\"if_nametoindex(\" \"%s\" \")\"), name_quoted_buf", 
    format=0x484e9b "if_nametoindex(%s)", format=0x484e9b "if_nametoindex(%s)", argstr=0x484eb0 "(\"if_nametoindex(\" \"%s\" \")\"), name_quoted_buf", func=0x484ede <__func__.6555> "get_ifname", size=36, 
    str=0x6f9a20 <res.6552> "if_nametoindex(\"\\x62\\x6c\\x69\\x6e\\x6") at xstring.h:36
#3  0x0000000000421596 in get_ifname (ifindex=6) at print_ifindex.c:31
#4  print_ifindex (ifindex=6) at print_ifindex.c:44
#5  0x0000000000427a7d in decode_ndmsg (tcp=0x712370, nlmsghdr=<optimized out>, family=<optimized out>, addr=140215372832688, len=60) at rtnl_neigh.c:86
#6  0x000000000041c5c8 in decode_netlink_route (tcp=tcp@entry=0x712370, nlmsghdr=nlmsghdr@entry=0x7fffffffe0b0, addr=addr@entry=140215372832688, len=len@entry=60) at netlink_route.c:120
...

  The problem in frame #3:

(gdb) f 3
#3  0x0000000000421596 in get_ifname (ifindex=6) at print_ifindex.c:31
31			xsprintf(res, INI_PFX "%s" INI_SFX, name_quoted_buf);
(gdb) info local
name_buf = "blink1\000\000\000\000\000\000\000\000\000"
name_quoted_buf = "\"\\x62\\x6c\\x69\\x6e\\x"
res = "if_nametoindex(\"\\x62\\x6c\\x69\\x6e\\x6"
res = "if_nametoindex(\"\\x62\\x6c\\x69\\x6e\\x6"
__func__ = "get_ifname"

  The fix could be to check the 'xflag' global variable and not print in hex
the name, but since all strings are printed in hex, the most likely fix should
be to rework the code in print_ifindex.c. This appears to be buggy:

# define IFNAME_QUOTED_SZ (sizeof(IFNAMSIZ) * 4 + 3)

as it most likely should mean:

# define IFNAME_QUOTED_SZ (IFNAMSIZ * 4 + 3)

because, otherwise it should result in:

sizeof(IFNAMSIZ) that is the same as sizeof(16) and return 4 (as literal
16 should default to type int). So, the code likely did already prepare for
printing in hex, but due to a mistake, used sizeof.