BUG/MINOR: htx: Preserve empty HTX messages with an unprocessed parsing error

This let a chance to HTX analyzers to handle the error and send the appropriate
response to the client.

This patch must be backported to 1.9.

include/common/htx.h

@@ -727,7 +727,7 @@ static inline struct htx *htx_from_buf(struct buffer *buf)
 /* Upate <buf> accordingly to the HTX message <htx> */
 static inline void htx_to_buf(struct htx *htx, struct buffer *buf)
 {
-	if (!htx->used) {
+	if (!htx->used && !(htx->flags & HTX_FL_PARSING_ERROR)) {
 		htx_reset(htx);
 		b_set_data(buf, 0);
 	}