Ensure associated netvc is still valid on SSL callbacks

Since SSL Callbacks are asynchronous in nature, it's possible the
associated NetVC is already freed causing a potential use-after-free
problem.

Author: sudheerv

iocore/net/SSLUtils.cc

@@ -1648,6 +1648,8 @@ SSLNetVCAccess(const SSL *ssl)
 
   ink_assert(dynamic_cast<SSLNetVConnection *>(static_cast<NetVConnection *>(SSL_get_ex_data(ssl, ssl_vc_index))));
 
+  ink_assert(netvc && netvc->ssl == ssl);
+
   return netvc;
 }