report: fix recent coverity warning.

[mhdawson]

Fix warning about dereferencing null env

[mhdawson]

This is the coverity report:

// External function to trigger a report, writing to file.
 945std::string TriggerNodeReport(Isolate* isolate,
 946                              const char* message,
 947                              const char* trigger,
 948                              const std::string& name,
 949                              Local<Value> error) {
    	1. assign_zero: Assigning: env = NULL.
 950  Environment* env = nullptr;
    	2. Condition isolate != NULL, taking false branch.
 951  if (isolate != nullptr) {
 952    env = Environment::GetCurrent(isolate);
 953  }
    	
CID 321974 (#1 of 1): Explicit null dereferenced (FORWARD_NULL)
3. var_deref_model: Passing null pointer env to TriggerNodeReport, which dereferences it. [[show details](https://scan9.scan.coverity.com/eventId=9118704-3&modelId=9118704-0&fileInstanceId=117525487&filePath=%2Fsrc%2Fnode_report.cc&fileStart=849&fileEnd=942)]
 954  return TriggerNodeReport(isolate, env, message, trigger, name, error);
 955}

[mhdawson]

Wrapping in a if env != nullptr seemed consistent with an existing similar case a few lines lower -

node/src/node_report.cc

Line 877 in 8f0f17e

if (env != nullptr) {

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/53067/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/53074/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/53324/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/53327/

[mhdawson]

Landed in 3d5e7cd