8352420: [ubsan] codeBuffer.cpp:984:27: runtime error: applying non-zero offset 18446744073709486080 to null pointer

Reviewed-by: kvn, mbaesken

Author: Doug Simon

src/hotspot/share/asm/codeBuffer.cpp

@@ -980,6 +980,7 @@ void CodeBuffer::take_over_code_from(CodeBuffer* cb) {
 
 void CodeBuffer::verify_section_allocation() {
   address tstart = _total_start;
+  if (tstart == nullptr) return;  // ignore not fully initialized buffer
   if (tstart == badAddress)  return;  // smashed by set_blob(nullptr)
   address tend   = tstart + _total_size;
   if (_blob != nullptr) {

src/hotspot/share/asm/codeBuffer.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2024, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2025, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -455,6 +455,8 @@ class CodeBuffer: public StackObj DEBUG_ONLY(COMMA private Scrubber) {
     _name            = name;
     _before_expand   = nullptr;
     _blob            = nullptr;
+    _total_start     = nullptr;
+    _total_size      = 0;
     _oop_recorder    = nullptr;
     _overflow_arena  = nullptr;
     _last_insn       = nullptr;