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 630684d3, 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.

src/interfaces/ecpg/pgtypeslib/dt_common.c

@@ -1682,6 +1682,7 @@ DecodePosixTimezone(char *str, int *tzp)
  *
  * The "lowstr" work buffer must have at least strlen(timestr) + MAXDATEFIELDS
  * bytes of space.  On output, field[] entries will point into it.
+ * The field[] and ftype[] arrays must have at least MAXDATEFIELDS entries.
  */
 int
 ParseDateTime(char *timestr, char *lowstr,
@@ -1695,9 +1696,9 @@ ParseDateTime(char *timestr, char *lowstr,
 	while (*(*endstr) != '\0')
 	{
 		/* Record start of current field */
-		field[nf] = lp;
 		if (nf >= MAXDATEFIELDS)
 			return -1;
+		field[nf] = lp;
 
 		/* leading digit? then date or time */
 		if (isdigit((unsigned char) *(*endstr)))