update: fix invalid parsing

src/misc/update_crypto.c

@@ -255,7 +255,7 @@ static size_t parse_signature_v4_packet( signature_packet_t *p_sig,
         }
         else
         {
-            if( p + 4 > max_pos )
+            if( ++p + 4 > max_pos )
                 return 0;
             i_subpacket_len = U32_AT(p);
             p += 4;