src: use SafeGetenv() for NODE_REDIRECT_WARNINGS

Mutations of the environment can invalidate pointers to environment
variables, so make `secure_getenv()` copy them out instead of returning
pointers.

This is the part of nodejs#11051 that
applies to be11fb4.
This part wasn't backported to 6.x when nodejs#11051 was backported
because the semver-minor introduction
of NODE_REDIRECT_WARNINGS hadn't been backported yet. Now that the
env var is backported, this last bit of nodejs#11051 is needed.

src/node.cc

@@ -207,7 +207,7 @@ bool config_preserve_symlinks = false;
 bool config_expose_internals = false;
 
 // Set in node.cc by ParseArgs when --redirect-warnings= is used.
-const char* config_warning_file;
+std::string config_warning_file;  // NOLINT(runtime/string)
 
 // process-relative uptime base, initialized at start-up
 static double prog_start_time;
@@ -4410,9 +4410,8 @@ void Init(int* argc,
   if (openssl_config.empty())
     SafeGetenv("OPENSSL_CONF", &openssl_config);
 
-  if (auto redirect_warnings = secure_getenv("NODE_REDIRECT_WARNINGS")) {
-    config_warning_file = redirect_warnings;
-  }
+  if (config_warning_file.empty())
+    SafeGetenv("NODE_REDIRECT_WARNINGS", &config_warning_file);
 
   // Parse a few arguments which are specific to Node.
   int v8_argc;

src/node_config.cc

@@ -49,11 +49,12 @@ void InitConfig(Local<Object> target,
   if (config_expose_internals)
     READONLY_BOOLEAN_PROPERTY("exposeInternals");
 
-  if (config_warning_file != nullptr) {
+  if (!config_warning_file.empty()) {
     Local<String> name = OneByteString(env->isolate(), "warningFile");
     Local<String> value = String::NewFromUtf8(env->isolate(),
-                                              config_warning_file,
-                                              v8::NewStringType::kNormal)
+                                              config_warning_file.data(),
+                                              v8::NewStringType::kNormal,
+                                              config_warning_file.size())
                                                 .ToLocalChecked();
     target->DefineOwnProperty(env->context(), name, value).FromJust();
   }

src/node_internals.h

@@ -52,7 +52,7 @@ extern bool config_expose_internals;
 // Set in node.cc by ParseArgs when --redirect-warnings= is used.
 // Used to redirect warning output to a file rather than sending
 // it to stderr.
-extern const char* config_warning_file;
+extern std::string config_warning_file;
 
 // Forward declaration
 class Environment;