BUG/MEDIUM: htx: Don't return the start-line if the HTX message is empty

In the function htx_get_stline(), NULL must be returned if the HTX message
doesn't contain any element.

This patch must be backported to 1.9.
diff --git a/include/common/htx.h b/include/common/htx.h
index 9cbc953..cd7db05 100644
--- a/include/common/htx.h
+++ b/include/common/htx.h
@@ -280,7 +280,7 @@
 {
 	struct htx_sl *sl = NULL;
 
-	if (htx->sl_off != -1)
+	if (htx->used && htx->sl_off != -1)
 		sl = ((void *)htx->blocks + htx->sl_off);
 
 	return sl;