Fix some compiler warnings with timestamp parsing in formatting.c

gcc-7 used with a sufficient optimization level complains about warnings
around do_to_timestamp() regarding the initialization and handling of
some of its variables.  Recent commits 66c74f8 and d589f94 made things
made the interface more confusing, so document which variables are
always expected and initialize properly the optional ones when they are
set.

Author: Andrey Lepikhov, Michael Paquier
Discussion: https://postgr.es/m/a7e28b83-27b1-4e1c-c76b-4268c4b785bc@postgrespro.ru

Author: michaelpq

src/backend/utils/adt/formatting.c

@@ -4128,7 +4128,7 @@ parse_datetime(text *date_txt, text *fmt, bool strict, Oid *typid,
 {
 	struct pg_tm tm;
 	fsec_t		fsec;
-	int			fprec = 0;
+	int			fprec;
 	uint32		flags;
 
 	do_to_timestamp(date_txt, fmt, strict, &tm, &fsec, &fprec, &flags, have_error);
@@ -4318,11 +4318,18 @@ do_to_timestamp(text *date_txt, text *fmt, bool std,
 	int			fmask;
 	bool		incache = false;
 
+	Assert(tm != NULL);
+	Assert(fsec != NULL);
+
 	date_str = text_to_cstring(date_txt);
 
 	ZERO_tmfc(&tmfc);
 	ZERO_tm(tm);
 	*fsec = 0;
+	if (fprec)
+		*fprec = 0;
+	if (flags)
+		*flags = 0;
 	fmask = 0;					/* bit mask for ValidateDate() */
 
 	fmt_len = VARSIZE_ANY_EXHDR(fmt);