Fix another date test

This may fix this crasher bug on windows:
https://bugzilla.gnome.org/show_bug.cgi?id=795040
However on linux it only fixes a warning so I can't be sure
until after the next nightly.

libgnucash/engine/qofquerycore.cpp

@@ -454,7 +454,7 @@ date_to_string (gpointer object, QofParam *getter)
 {
     Timespec ts = ((query_date_getter)getter->param_getfcn)(object, getter);
 
-    if (ts.tv_sec || ts.tv_nsec)
+    if (ts.tv_sec != INT64_MAX)
         return g_strdup (gnc_print_date (ts));
 
     return NULL;