quic: guard against null impl_ in UpdateDataStats

When a QUIC session's handshake fails or the connection is
terminated early, UpdateDataStats() can be called before impl_
has been initialized, leading to a null pointer dereference
and SIGSEGV.

Add an early return when impl_ is nullptr to prevent the crash.

Fixes: #62057

Co-Authored-By: Claude <noreply@anthropic.com>

Author: zerone0x

src/quic/session.cc

@@ -2232,6 +2232,7 @@ void Session::ExtendOffset(size_t amount) {
 }
 
 void Session::UpdateDataStats() {
+  if (!impl_) return;
   Debug(this, "Updating data stats");
   auto& stats_ = impl_->stats_;
   ngtcp2_conn_info info;