[DevTools] Avoid dereferencing null pointer

third_party/WebKit/Source/platform/v8_inspector/V8InspectorSessionImpl.cpp

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "platform/v8_inspector/V8InspectorSessionImpl.h"
 
 #include "platform/inspector_protocol/Parser.h"
 #include "platform/v8_inspector/InjectedScript.h"
 #include "platform/v8_inspector/InspectedContext.h"
 #include "platform/v8_inspector/RemoteObjectId.h"
@@ skipping 121 matching lines @@
 }
 
 InjectedScript* V8InspectorSessionImpl::findInjectedScript(ErrorString* errorString, int contextId)
 {
     if (!contextId) {
         *errorString = "Cannot find context with specified id";
         return nullptr;
     }
 
     const V8DebuggerImpl::ContextByIdMap* contexts = m_debugger->contextGroup(m_contextGroupId);
-    auto contextsIt = contexts ? contexts->find(contextId) : contexts->end();
+    if (!contexts) {
+        *errorString = "Cannot find context with specified id";
+        return nullptr;
+    }
+
+    auto contextsIt = contexts->find(contextId);
     if (contextsIt == contexts->end()) {
         *errorString = "Cannot find context with specified id";
         return nullptr;
     }
 
     const std::unique_ptr<InspectedContext>& context = contextsIt->second;
     if (!context->getInjectedScript()) {
         context->createInjectedScript();
         if (!context->getInjectedScript()) {
             *errorString = "Cannot access specified execution context";
@@ skipping 142 matching lines @@
     m_debuggerAgent->resume(&errorString);
 }
 
 void V8InspectorSessionImpl::stepOver()
 {
     ErrorString errorString;
     m_debuggerAgent->stepOver(&errorString);
 }
 
 } // namespace blink