CVE-2014-8132: Fixup error path in ssh_packet_kexinit()

Before this change, dangling pointers can be unintentionally left in the
respective next_crypto kex methods slots.  Ensure to set all slots to
NULL in the error-out path.

Signed-off-by: Jon Simons <jon@jonsimons.org>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/kex.c

@@ -368,7 +368,7 @@ SSH_PACKET_CALLBACK(ssh_packet_kexinit){
     for (i = 0; i < KEX_METHODS_SIZE; i++) {
         str = buffer_get_ssh_string(packet);
         if (str == NULL) {
-          break;
+          goto error;
         }
 
         rc = buffer_add_ssh_string(session->in_hashbuf, str);
@@ -443,6 +443,11 @@ SSH_PACKET_CALLBACK(ssh_packet_kexinit){
 error:
     ssh_string_free(str);
     for (i = 0; i < SSH_KEX_METHODS; i++) {
+        if (server_kex) {
+            session->next_crypto->client_kex.methods[i] = NULL;
+        } else { /* client */
+            session->next_crypto->server_kex.methods[i] = NULL;
+        }
         SAFE_FREE(strings[i]);
     }