MINOR: htx: Store start-line block's position instead of address of its payload

Nothing much to say. This change is just mandatory to consider 1xx informational
messages as part of a response.
diff --git a/src/http_htx.c b/src/http_htx.c
index 471f1ab..e6332f7 100644
--- a/src/http_htx.c
+++ b/src/http_htx.c
@@ -40,7 +40,7 @@
 
 		if (type == HTX_BLK_REQ_SL || type == HTX_BLK_RES_SL) {
 			sl = htx_get_blk_ptr(htx, blk);
-			htx->sl_off = blk->addr;
+			htx->sl_pos = pos;
 			break;
 		}
 
@@ -194,7 +194,7 @@
 		struct htx_blk *blk = htx_get_blk(htx, pos);
 		enum htx_blk_type type = htx_get_blk_type(blk);
 
-		if (htx->sl_off == blk->addr) {
+		if (htx->sl_pos == pos) {
 			if (!htx_replace_stline(htx, blk, p1, p2, p3))
 				return 0;
 			return 1;