Fix filestream checking in output_json.cpp (#1063)

Summary:
The code is missing several checks for file stream validity, which causes the traced application to crash when it is unable to create a file.

Pull Request resolved: #1063

Reviewed By: aaronenyeshi

Differential Revision: D71902855

Pulled By: sraikund16

fbshipit-source-id: 1b139b4e154cc3cc017a46a705433bc0bd6eebdb

Author: Ivan Timokhin

libkineto/src/output_json.cpp

@@ -118,6 +118,10 @@ void ChromeTraceLogger::metadataToJSON(
 void ChromeTraceLogger::handleTraceStart(
     const std::unordered_map<std::string, std::string>& metadata,
     const std::string& device_properties) {
+  if (!traceOf_) {
+    return;
+  }
+
   traceOf_ << fmt::format(
       R"JSON(
 {{
@@ -556,10 +560,16 @@ void ChromeTraceLogger::handleActivity(const libkineto::ITraceActivity& op) {
 
 void ChromeTraceLogger::handleGenericActivity(
     const libkineto::GenericTraceActivity& op) {
+  if (!traceOf_) {
+    return;
+  }
   handleActivity(op);
 }
 
 void ChromeTraceLogger::handleGenericLink(const ITraceActivity& act) {
+  if (!traceOf_) {
+    return;
+  }
   static struct {
     int type;
     char name[16];