Check snprintf return value

When possibility of truncating is expected or we are sure it will never
happen, we should check the return value. If the return value is checked
and program is branched, the GCC warning is not issued.
See https://bugzilla.redhat.com/show_bug.cgi?id=1431678#c3

Addressing:
/home/jcerny/openscap/src/XCCDF/result.c: In function ‘xccdf_rule_result_set_time_current’:
/home/jcerny/openscap/src/XCCDF/result.c:1578:46: warning: ‘%02d’ directive output may be truncated writing between 2 and 11 bytes into a region of size between 8 and 15 [-Wformat-truncation=]
 1578 |  snprintf(timestamp, sizeof(timestamp), "%4d-%02d-%02dT%02d:%02d:%02d",
      |                                              ^~~~
/home/jcerny/openscap/src/XCCDF/result.c:1578:41: note: directive argument in the range [-2147483647, 2147483647]
 1578 |  snprintf(timestamp, sizeof(timestamp), "%4d-%02d-%02dT%02d:%02d:%02d",
      |                                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/jcerny/openscap/src/XCCDF/result.c:1578:2: note: ‘snprintf’ output between 20 and 72 bytes into a destination of size 20
 1578 |  snprintf(timestamp, sizeof(timestamp), "%4d-%02d-%02dT%02d:%02d:%02d",
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1579 |   1900 + lt->tm_year, 1 + lt->tm_mon, lt->tm_mday,
      |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1580 |   lt->tm_hour, lt->tm_min, lt->tm_sec);
      |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/jcerny/openscap/src/XCCDF/result.c: In function ‘xccdf_result_set_start_time_current’:
/home/jcerny/openscap/src/XCCDF/result.c:1578:46: warning: ‘%02d’ directive output may be truncated writing between 2 and 11 bytes into a region of size between 8 and 15 [-Wformat-truncation=]
 1578 |  snprintf(timestamp, sizeof(timestamp), "%4d-%02d-%02dT%02d:%02d:%02d",
      |                                              ^~~~
/home/jcerny/openscap/src/XCCDF/result.c:1578:41: note: directive argument in the range [-2147483647, 2147483647]
 1578 |  snprintf(timestamp, sizeof(timestamp), "%4d-%02d-%02dT%02d:%02d:%02d",
      |                                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/jcerny/openscap/src/XCCDF/result.c:1578:2: note: ‘snprintf’ output between 20 and 72 bytes into a destination of size 20
 1578 |  snprintf(timestamp, sizeof(timestamp), "%4d-%02d-%02dT%02d:%02d:%02d",
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1579 |   1900 + lt->tm_year, 1 + lt->tm_mon, lt->tm_mday,
      |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1580 |   lt->tm_hour, lt->tm_min, lt->tm_sec);
      |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/jcerny/openscap/src/XCCDF/result.c: In function ‘xccdf_result_set_end_time_current’:
/home/jcerny/openscap/src/XCCDF/result.c:1578:46: warning: ‘%02d’ directive output may be truncated writing between 2 and 11 bytes into a region of size between 8 and 15 [-Wformat-truncation=]
 1578 |  snprintf(timestamp, sizeof(timestamp), "%4d-%02d-%02dT%02d:%02d:%02d",
      |                                              ^~~~
/home/jcerny/openscap/src/XCCDF/result.c:1578:41: note: directive argument in the range [-2147483647, 2147483647]
 1578 |  snprintf(timestamp, sizeof(timestamp), "%4d-%02d-%02dT%02d:%02d:%02d",
      |                                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/jcerny/openscap/src/XCCDF/result.c:1578:2: note: ‘snprintf’ output between 20 and 72 bytes into a destination of size 20
 1578 |  snprintf(timestamp, sizeof(timestamp), "%4d-%02d-%02dT%02d:%02d:%02d",
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1579 |   1900 + lt->tm_year, 1 + lt->tm_mon, lt->tm_mday,
      |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1580 |   lt->tm_hour, lt->tm_min, lt->tm_sec);
      |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Author: jan-cerny

src/XCCDF/result.c

@@ -1575,9 +1575,12 @@ static inline const char *_get_timestamp(void)
 
 	tm = time(NULL);
 	lt = localtime(&tm);
-	snprintf(timestamp, sizeof(timestamp), "%4d-%02d-%02dT%02d:%02d:%02d",
+	int ret = snprintf(timestamp, sizeof(timestamp), "%4d-%02d-%02dT%02d:%02d:%02d",
 		1900 + lt->tm_year, 1 + lt->tm_mon, lt->tm_mday,
 		lt->tm_hour, lt->tm_min, lt->tm_sec);
+	if (ret < 0) {
+		return NULL;
+	}
 	return timestamp;
 }