Fix array overrun in ecpg's version of ParseDateTime().

 Fix array overrun in ecpg's version of ParseDateTime().
 
 The code wrote a value into the caller's field[] array before checking
 to see if there was room, which of course is backwards.  Per report from
 Michael Paquier.
 
 I fixed the equivalent bug in the backend's version of this code way back
 in 630684d3a130bb93, but failed to think about ecpg's copy.  Fortunately
 this doesn't look like it would be exploitable for anything worse than a
 core dump: an external attacker would have no control over the single word
 that gets written.