[BUG] Fix NULL pointer dereference in stats_check_uri_auth(), v2

Recent "struct chunk rework" introduced a NULL pointer dereference
and now haproxy segfaults if auth is required for stats but not found.

The reason is that size_t cannot store negative values, but current
code assumes that "len < 0" == uninitialized.

This patch fixes it.
diff --git a/src/proto_http.c b/src/proto_http.c
index 4638d09..8698594 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -4596,8 +4596,7 @@
 			int len = txn->hdr_idx.v[cur_idx].len;
 			if (len > 14 &&
 			    !strncasecmp("Authorization:", h, 14)) {
-				txn->auth_hdr.str = h;
-				txn->auth_hdr.len = len;
+				chunk_initlen(&txn->auth_hdr, h, 0, len);
 				break;
 			}
 			h += len + txn->hdr_idx.v[cur_idx].cr + 1;