[tcp_proxy] Add nullptr checks to watermark asserts (#40034)

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Commit Message: Add nullptr checks into watermark asserts in tcp proxy
to avoid potential segfaults.
Additional Description:
Risk Level: Low
Testing:
Docs Changes:
Release Notes:
Platform Specific Features:

---------

Signed-off-by: Kateryna Nezdolii <kateryna.nezdolii@gmail.com>

Author: nezdolik

source/common/tcp_proxy/tcp_proxy.cc

@@ -436,8 +436,10 @@ void Filter::UpstreamCallbacks::onEvent(Network::ConnectionEvent event) {
 }
 
 void Filter::UpstreamCallbacks::onAboveWriteBufferHighWatermark() {
+  // In case when upstream connection is draining `parent_` will be set to nullptr.
   // TCP Tunneling may call on high/low watermark multiple times.
-  ASSERT(parent_->config_->tunnelingConfigHelper() || !on_high_watermark_called_);
+  ASSERT(parent_ == nullptr || parent_->config_->tunnelingConfigHelper() ||
+         !on_high_watermark_called_);
   on_high_watermark_called_ = true;
 
   if (parent_ != nullptr) {
@@ -447,8 +449,10 @@ void Filter::UpstreamCallbacks::onAboveWriteBufferHighWatermark() {
 }
 
 void Filter::UpstreamCallbacks::onBelowWriteBufferLowWatermark() {
+  // In case when upstream connection is draining `parent_` will be set to nullptr.
   // TCP Tunneling may call on high/low watermark multiple times.
-  ASSERT(parent_->config_->tunnelingConfigHelper() || on_high_watermark_called_);
+  ASSERT(parent_ == nullptr || parent_->config_->tunnelingConfigHelper() ||
+         on_high_watermark_called_);
   on_high_watermark_called_ = false;
 
   if (parent_ != nullptr) {