Avoid out-of-range error for negative time on `MRB_TIME_T_UINT`.

On platforms where `time_t` is unsigned, negative time can be a result
of integer casting. Out-of-range error is too strict for those cases.
This fix does not address wrong time value in `MRB_WORD_BOXING`. It will
be addressed later (by introducing "big" integers).

mrbgems/mruby-time/src/time.c

@@ -257,7 +257,7 @@ mrb_to_time_t(mrb_state *mrb, mrb_value obj, time_t *usec)
         mrb_int i = mrb_int(mrb, obj);
 
         if ((MRB_INT_MAX > MRB_TIME_MAX && i > 0 && i > (mrb_int)MRB_TIME_MAX) ||
-            (MRB_TIME_MIN > MRB_INT_MIN && MRB_TIME_MIN > i)) {
+            (!MRB_TIME_T_UINT && MRB_TIME_MIN > MRB_INT_MIN && MRB_TIME_MIN > i)) {
           goto out_of_range;
         }