Fix corner case of os.time().

Thanks to Temir Galeev. LuaJIT#1470

(cherry picked from commit 72d2061)

This patch fixes invalid os.time(t) behavior, when it handles -1 time value
as a fail of the libc mktime() regardless of the errno value.

Temir Galeev:
* added the description and the test for the patch

Part of tarantool/tarantool#12480

Author: Mike Pall

src/lib_os.c

@@ -242,9 +242,10 @@ 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))
+  if (t == (time_t)(-1) && errno != 0)
     lua_pushnil(L);
   else
     lua_pushnumber(L, (lua_Number)t);

test/tarantool-tests/lj-1470-os-time-epoch-minus-1s.test.lua

@@ -0,0 +1,13 @@
+local tap = require('tap')
+
+-- The test file demonstrates os.time() fail to return -1 time
+-- value.
+-- See also: https://github.com/LuaJIT/LuaJIT/issues/1470.
+local test = tap.test('lj-1470-os-time-epoch-minus-1s')
+
+test:plan(1)
+
+local minus_1s_time = os.date("*t", -1)
+test:is(os.time(minus_1s_time), -1, 'correct os.time()')
+
+test:done(true)