sprintf/snprintf format string overflow warnings

[cranderson]

When compiling on Fedora 31:

gcc -DHAVE_CONFIG_H -I. -I.. -Wall -Wextra -Wno-sign-compare -DIPV6 -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -c -o fping-fping.o test -f 'fping.c' || echo './'fping.c
make[2]: Leaving directory '/builddir/build/BUILD/fping-4.2/src'
fping.c: In function 'sprint_tm':
fping.c:2599:28: warning: '__builtin___sprintf_chk' may write a terminating nul past the end of the destination [-Wformat-overflow=]
2599 | sprintf(buf, "%d.%d", t / 100, (t % 100) / 10);
| ^
In file included from /usr/include/stdio.h:867,
from fping.c:44:
/usr/include/bits/stdio2.h:36:10: note: '__builtin___sprintf_chk' output between 4 and 11 bytes into a destination of size 10
36 | return __builtin___sprintf_chk (__s, __USE_FORTIFY_LEVEL - 1,
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
37 | __bos (__s), __fmt, __va_arg_pack ());
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
fping.c:2595:26: warning: '%02d' directive writing 2 bytes into a region of size between 1 and 8 [-Wformat-overflow=]
2595 | sprintf(buf, "%d.%02d", t / 100, t % 100);
| ^~~~
fping.c:2595:22: note: directive argument in the range [0, 99]
2595 | sprintf(buf, "%d.%02d", t / 100, t % 100);
| ^~~~~~~~~
In file included from /usr/include/stdio.h:867,
from fping.c:44:
/usr/include/bits/stdio2.h:36:10: note: '__builtin___sprintf_chk' output between 5 and 12 bytes into a destination of size 10
36 | return __builtin___sprintf_chk (__s, __USE_FORTIFY_LEVEL - 1,
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
37 | __bos (__s), __fmt, __va_arg_pack ());
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
fping.c: In function 'add_name':
fping.c:2340:82: warning: ')' directive output may be truncated writing 1 byte into a region of size between 0 and 510 [-Wformat-truncation=]
2340 | snprintf(nameaddrbuf, sizeof(nameaddrbuf) / sizeof(char), "%s (%s)", printname, addrbuf);
| ^
In file included from /usr/include/stdio.h:867,
from fping.c:44:
/usr/include/bits/stdio2.h:67:10: note: '__builtin___snprintf_chk' output between 4 and 514 bytes into a destination of size 512
67 | return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
68 | __bos (__s), __fmt, __va_arg_pack ());
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~