Incorrect snprintf invocation with static buffer

[derickr]

When compiling timelib with -O2 and GCC 8.1, the following compile warning is shown:

src/third_party/timelib-2018.01alpha1/parse_zoneinfo.c: In function ‘create_zone_index.constprop’:
src/third_party/timelib-2018.01alpha1/parse_zoneinfo.c:118:40: warning: ‘__builtin___snprintf_chk’ output may be truncated before the last format character [-Wformat-truncation=]
  snprintf(fname, sizeof(fname), "%s%s%s", directory, TIMELIB_DIR_SEPARATOR, timezone /* canonical_tzname(timezone) */);
                                        ^
In file included from /usr/include/stdio.h:862,
                 from src/third_party/timelib-2018.01alpha1/timelib_private.h:70,
                 from src/third_party/timelib-2018.01alpha1/parse_zoneinfo.c:27:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:64:10: note: ‘__builtin___snprintf_chk’ output 2 or more bytes (assuming 4097) into a destination of size 4096
   return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        __bos (__s), __fmt, __va_arg_pack ());
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~