Fix: Backtraces will NULL Addresses (AMReX-Codes#3502)

## Summary

The handing of invalid stack addresses in backtraces was incomplete,
failing to run for stack addresses down a nullptr address with the
error:
```
sh: -c: line 0: syntax error near unexpected token `('
sh: -c: line 0: `/usr/bin/addr2line -Cpfie /gpfs/alpine/aph114/proj-shared/huebl/2023-08-21_ms-debugging-rz/002_btd_patch_pr4208/./warpx.rz (nil)'
```

- [x] seen on Summit (OLCF)
- [ ] tested to fix the issue

## Additional background

## Checklist

The proposed changes:
- [x] fix a bug or incorrect behavior in AMReX
- [ ] add new capabilities to AMReX
- [ ] changes answers in the test suite to more than roundoff level
- [ ] are likely to significantly affect the results of downstream AMReX
users
- [ ] include documentation in the code and/or rst files, if appropriate

Src/Base/AMReX_BLBackTrace.cpp

@@ -239,10 +239,12 @@ BLBackTrace::print_backtrace_info (FILE* f)
  std::fprintf(f, "%2d: %s\n", i, strings[i]);
 
 #if !defined(AMREX_USE_OMP) || !defined(__INTEL_COMPILER)
+ const bool stack_ptr_not_null = (bt_buffer[i] != nullptr);
+
  std::string addr2line_result;
  bool try_addr2line = false;
  if (amrex::system::call_addr2line && have_eu_addr2line) {
- if (bt_buffer[i] != nullptr) {
+ if (stack_ptr_not_null) {
  char print_buff[32];
  std::snprintf(print_buff,sizeof(print_buff),"%p",bt_buffer[i]);
  const std::string full_cmd = eu_cmd + " " + print_buff;
@@ -255,7 +257,7 @@ BLBackTrace::print_backtrace_info (FILE* f)
  try_addr2line = true;
  }
  if (try_addr2line && amrex::system::call_addr2line && have_addr2line &&
- !amrex::system::exename.empty())
+ !amrex::system::exename.empty() && stack_ptr_not_null)
  {
  addr2line_result.clear();
  const std::string line = strings[i];
@@ -307,10 +309,11 @@ BLBackTrace::print_backtrace_info (FILE* f)
 
  for (int i = 0; i < nentries; ++i) {
  Dl_info info;
+ const bool stack_ptr_not_null = (bt_buffer[i] != nullptr);
  if (dladdr(bt_buffer[i], &info))
  {
  std::string line;
- if (amrex::system::call_addr2line && have_atos) {
+ if (amrex::system::call_addr2line && have_atos && stack_ptr_not_null) {
  char print_buff[32];
  std::snprintf(print_buff,sizeof(print_buff),"%p",bt_buffer[i]);
  const std::string full_cmd = cmd + " " + print_buff;