MT#57436 intimeToXml: properly manage the buf size and `snprintf()`

Give an exact size of the buf to the `snprintf()` and also treat the
returned value from it to eliminate this warning:

     src/XmlRpcValue.cpp: In member function 'std::string XmlRpc::XmlRpcValue::timeToXml() const':
     src/XmlRpcValue.cpp:404:53: warning: '%02d' directive output may be truncated writing between 2 and 11 bytes into a region of size between 0 and 7 [-Wformat-truncation=]
       404 |     snprintf(buf, sizeof(buf)-1, "%04d%02d%02dT%02d:%02d:%02d",
           |                                                     ^~~~
     In file included from /usr/include/stdio.h:867,
                      from /usr/include/c++/10/cstdio:42,
                      from /usr/include/c++/10/ext/string_conversions.h:43,
                      from /usr/include/c++/10/bits/basic_string.h:6545,
                      from /usr/include/c++/10/string:55,
                      from src/XmlRpcValue.h:14,
                      from src/XmlRpcValue.cpp:2:
     /usr/include/x86_64-linux-gnu/bits/stdio2.h:67:35: note: '__builtin___snprintf_chk' output between 18 and 70 bytes into a destination of size 19
        67 |   return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
           |          ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        68 |        __bos (__s), __fmt, __va_arg_pack ());
           |        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Change-Id: Ic545bbc942715f433610f49286ee9d0f92ee25f6

apps/xmlrpc2di/xmlrpc++/src/XmlRpcValue.cpp

@@ -400,10 +400,13 @@ namespace XmlRpc {
   std::string XmlRpcValue::timeToXml() const
   {
     struct tm* t = _value.asTime;
-    char buf[20];
-    snprintf(buf, sizeof(buf)-1, "%4d%02d%02dT%02d:%02d:%02d",
-             t->tm_year, t->tm_mon, t->tm_mday, t->tm_hour, t->tm_min, t->tm_sec);
-    buf[sizeof(buf)-1] = 0;
+    char buf[18];
+
+    if (snprintf(buf, sizeof(buf), "%04d%02d%02dT%02d:%02d:%02d",
+                (1900 + t->tm_year), t->tm_mon, t->tm_mday, t->tm_hour, t->tm_min, t->tm_sec) < 0)
+    {
+      XmlRpcUtil::log(2,"timeToXml: issues while trying to write data.");
+    }
 
     std::string xml = VALUE_TAG;
     xml += DATETIME_TAG;