Use snprintf not sprintf in pg_waldump's timestamptz_to_str.

This could only cause an issue if strftime returned a ridiculously
long timezone name, which seems unlikely; and it wouldn't qualify
as a security problem even then, since pg_waldump (nee pg_xlogdump)
is a debug tool not part of the server.  But gcc 8 has started issuing
warnings about it, so let's use snprintf and be safe.

Backpatch to 9.3 where this code was added.

Discussion: https://postgr.es/m/21789.1529170195@sss.pgh.pa.us

src/bin/pg_waldump/compat.c

@@ -58,7 +58,8 @@ timestamptz_to_str(TimestampTz dt)
 	strftime(ts, sizeof(ts), "%Y-%m-%d %H:%M:%S", ltime);
 	strftime(zone, sizeof(zone), "%Z", ltime);
 
-	sprintf(buf, "%s.%06d %s", ts, (int) (dt % USECS_PER_SEC), zone);
+	snprintf(buf, sizeof(buf), "%s.%06d %s",
+			 ts, (int) (dt % USECS_PER_SEC), zone);
 
 	return buf;
 }