Avoid statically allocating gmtsub()'s timezone workspace.

localtime.c's "struct state" is a rather large object, ~23KB.  We were
statically allocating one for gmtsub() to use to represent the GMT
timezone, even though that function is not at all heavily used and is
never reached in most backends.  Let's malloc it on-demand, instead.

This does pose the question of how to handle a malloc failure, but
there's already a well-defined error report convention here, ie
set errno and return NULL.

We have but one caller of pg_gmtime in HEAD, and two in back branches,
neither of which were troubling to check for error.  Make them do so.
The possible errors are sufficiently unlikely (out-of-range timestamp,
and now malloc failure) that I think elog() is adequate.

Back-patch to all supported branches to keep our copies of the IANA
timezone code in sync.  This particular change is in a stanza that
already differs from upstream, so it's a wash for maintenance purposes
--- but only as long as we keep the branches the same.

Discussion: https://postgr.es/m/20181015200754.7y7zfuzsoux2c4ya@alap3.anarazel.de

src/backend/utils/adt/timestamp.c

@@ -2008,6 +2008,9 @@ GetEpochTime(struct pg_tm *tm)
 
 	t0 = pg_gmtime(&epoch);
 
+	if (t0 == NULL)
+		elog(ERROR, "could not convert epoch to timestamp: %m");
+
 	tm->tm_year = t0->tm_year;
 	tm->tm_mon = t0->tm_mon;
 	tm->tm_mday = t0->tm_mday;

src/timezone/localtime.c

@@ -1328,13 +1328,14 @@ gmtsub(pg_time_t const *timep, int32 offset, struct pg_tm *tmp)
 	struct pg_tm *result;
 
 	/* GMT timezone state data is kept here */
-	static struct state gmtmem;
-	static bool gmt_is_set = false;
-#define gmtptr		(&gmtmem)
+	static struct state *gmtptr = NULL;
 
-	if (!gmt_is_set)
+	if (gmtptr == NULL)
 	{
-		gmt_is_set = true;
+		/* Allocate on first use */
+		gmtptr = (struct state *) malloc(sizeof(struct state));
+		if (gmtptr == NULL)
+			return NULL;		/* errno should be set by malloc */
 		gmtload(gmtptr);
 	}
 	result = timesub(timep, offset, gmtptr, tmp);