Replace static buf with palloc in str_time()

The function is used only once in the startup process, so the leak
into current memory context is harmless.

This is a tiny step in making the server thread-safe.

Reviewed-by: Robert Haas
Discussion: https://www.postgresql.org/message-id/7f86e06a-98c5-4ce3-8ec9-3885c8de0358@iki.fi

src/backend/access/transam/xlog.c

@@ -5168,9 +5168,9 @@ BootStrapXLOG(uint32 data_checksum_version)
 static char *
 str_time(pg_time_t tnow)
 {
-	static char buf[128];
+	char	   *buf = palloc(128);
 
-	pg_strftime(buf, sizeof(buf),
+	pg_strftime(buf, 128,
 				"%Y-%m-%d %H:%M:%S %Z",
 				pg_localtime(&tnow, log_timezone));