8337784: Fix simple -Wzero-as-null-pointer-constant warnings in linux/posix code

Reviewed-by: stefank, shade

Author: Kim Barrett

src/hotspot/os/linux/os_linux.cpp

@@ -864,7 +864,7 @@ static void *thread_native_entry(Thread *thread) {
   log_info(os, thread)("Thread finished (tid: " UINTX_FORMAT ", pthread id: " UINTX_FORMAT ").",
     os::current_thread_id(), (uintx) pthread_self());
 
-  return 0;
+  return nullptr;
 }
 
 // On Linux, glibc places static TLS blocks (for __thread variables) on
@@ -4449,7 +4449,7 @@ void os::init(void) {
   check_pax();
 
   // Check the availability of MADV_POPULATE_WRITE.
-  FLAG_SET_DEFAULT(UseMadvPopulateWrite, (::madvise(0, 0, MADV_POPULATE_WRITE) == 0));
+  FLAG_SET_DEFAULT(UseMadvPopulateWrite, (::madvise(nullptr, 0, MADV_POPULATE_WRITE) == 0));
 
   os::Posix::init();
 }

src/hotspot/os/posix/perfMemory_posix.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2024, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2012, 2021 SAP SE. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
@@ -1329,7 +1329,7 @@ void PerfMemory::attach(int vmid, char** addrp, size_t* sizep, TRAPS) {
 //
 void PerfMemory::detach(char* addr, size_t bytes) {
 
-  assert(addr != 0, "address sanity check");
+  assert(addr != nullptr, "address sanity check");
   assert(bytes > 0, "capacity sanity check");
 
   if (PerfMemory::contains(addr) || PerfMemory::contains(addr + bytes - 1)) {

src/hotspot/os/posix/signals_posix.cpp

@@ -1719,7 +1719,7 @@ static int SR_initialize() {
   struct sigaction act;
   char *s;
   // Get signal number to use for suspend/resume
-  if ((s = ::getenv("_JAVA_SR_SIGNUM")) != 0) {
+  if ((s = ::getenv("_JAVA_SR_SIGNUM")) != nullptr) {
     int sig;
     bool result = parse_integer(s, &sig);
     if (result && sig > MAX2(SIGSEGV, SIGBUS) &&  // See 4355769.
@@ -1742,7 +1742,7 @@ static int SR_initialize() {
   pthread_sigmask(SIG_BLOCK, nullptr, &act.sa_mask);
   remove_error_signals_from_set(&(act.sa_mask));
 
-  if (sigaction(PosixSignals::SR_signum, &act, 0) == -1) {
+  if (sigaction(PosixSignals::SR_signum, &act, nullptr) == -1) {
     return -1;
   }