In qof_scan_date returns valid date when day and month are 0

If you start typing a date in the register with the enable autoreadonly
option starting with a 0, gdate complains when trying to create a valid
date.

libgnucash/engine/gnc-date.cpp

@@ -936,6 +936,9 @@ qof_scan_date_internal (const char *buff, int *day, int *month, int *year,
 
     g_free (dupe);
 
+    if ((imonth == 0) || (iday == 0))
+        return FALSE;
+
     if ((12 < imonth) || (31 < iday))
     {
         /*