[BUG] check_post: limit analysis to the buffer length

If "balance url_param XXX check_post" is used, we must bound the
number of bytes analysed to the buffer's length.
diff --git a/src/backend.c b/src/backend.c
index f4e6110..70201ce 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -289,6 +289,9 @@
 		len = chunk;
 	}
 
+	if (len > req->l - body)
+		len = req->l - body;
+
 	p = params;
 
 	while (len > plen) {