Make check in os.time() consistent.

Thanks to Temir Galeev.

(cherry picked from commit 295d45f)

This patch prevents os.time() from returning nil, when machine time
is -1 and some error occurs before the os.time() call. In fact,
this is a theoretical case, so no tests were added.

Temir Galeev:
* added the description for the patch

Part of tarantool/tarantool#12480

Author: Mike Pall

src/lib_os.c

@@ -229,6 +229,7 @@ LJLIB_CF(os_date)
 LJLIB_CF(os_time)
 {
   time_t t;
+  errno = 0;
   if (lua_isnoneornil(L, 1)) {  /* called without args? */
     t = time(NULL);  /* get current time */
   } else {
@@ -242,7 +243,6 @@ LJLIB_CF(os_time)
     ts.tm_mon = getfield(L, "month", -1) - 1;
     ts.tm_year = getfield(L, "year", -1) - 1900;
     ts.tm_isdst = getboolfield(L, "isdst");
-    errno = 0;
     t = mktime(&ts);
   }
   if (t == (time_t)(-1) && errno != 0)