leapdb: fix ordered comparison against NULL pointer

fgets returns either a valid pointer with the same value as its first
argument or NULL on error or EOF.

GCC 12.2.0 -Wextra warns against relational comparison of the return
value:

  leapdb.c:127:38: warning: ordered comparison of pointer with integer zero [-Wextra]

For clarity, and because the C standard doesn't mandate that valid pointers
have to compare greater than the null pointer constant, replace the
relational expression with an equality expression

leapdb.c

@@ -124,7 +124,7 @@ get_list_leap(time_t when, int *tai_offset)
   /* leap-seconds.list timestamps are relative to 1 Jan 1900, 00:00:00 */
   when1900 = (int64_t)when + LEAP_SEC_LIST_OFFSET;
 
-  while (fgets(line, sizeof line, f) > 0) {
+  while (fgets(line, sizeof line, f)) {
     int64_t lsl_when;
     int lsl_tai_offset;
     char *p;