BUG/MEDIUM: http: Prevent replace-header from overwriting a buffer

This is the same fix as which concerning the redirect rules (0d94576c).

The buffer used to expand the <replace-fmt> argument must be protected to
prevent it being overwritten during build_logline() execution (the function used
to expand the format string).

This patch should be backported in 1.7, 1.6 and 1.5. It relies on commit b686afd
("MINOR: chunks: implement a simple dynamic allocator for trash buffers") for
the trash allocator, which has to be backported as well.

src/proto_http.c

@@ -3419,13 +3419,22 @@ static int http_transform_header(struct stream* s, struct http_msg *msg,
                                  struct list *fmt, struct my_regex *re,
                                  int action)
 {
-	struct chunk *replace = get_trash_chunk();
+	struct chunk *replace;
+	int ret = -1;
+
+	replace = alloc_trash_chunk();
+	if (!replace)
+		goto leave;
 
 	replace->len = build_logline(s, replace->str, replace->size, fmt);
 	if (replace->len >= replace->size - 1)
-		return -1;
+		goto leave;
 
-	return http_transform_header_str(s, msg, name, name_len, replace->str, re, action);
+	ret = http_transform_header_str(s, msg, name, name_len, replace->str, re, action);
+
+  leave:
+	free_trash_chunk(replace);
+	return ret;
 }
 
 /* Executes the http-request rules <rules> for stream <s>, proxy <px> and