Fix NPN protocol name list validation

Since 50932c4 "PACKETise ServerHello processing",
ssl_next_proto_validate() incorrectly allows empty protocol name.
draft-agl-tls-nextprotoneg-04[1] says "Implementations MUST ensure that
the empty string is not included and that no byte strings are
truncated."
This patch restores the old correct behavior.

[1] https://tools.ietf.org/html/draft-agl-tls-nextprotoneg-04

Reviewed-by: Emilia Käsper <emilia@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>

ssl/t1_lib.c

diff --git a/ssl/t1_lib.c b/ssl/t1_lib.c
index 6363348..3082a59 100644
--- a/ssl/t1_lib.c
+++ b/ssl/t1_lib.c
@@ -2339,11 +2339,11 @@
  */
 static char ssl_next_proto_validate(PACKET *pkt)
 {
-    unsigned int len;
+    PACKET tmp_protocol;
 
     while (PACKET_remaining(pkt)) {
-        if (!PACKET_get_1(pkt, &len)
-                || !PACKET_forward(pkt, len))
+        if (!PACKET_get_length_prefixed_1(pkt, &tmp_protocol)
+                || PACKET_remaining(&tmp_protocol) == 0)
             return 0;
     }