Cherry-pick cd62341. rdar://124104099

    Web Inspector: Crash in Inspector::InspectorDebuggerAgent::didScheduleAsyncCall on knowyourmeme.com
    https://bugs.webkit.org/show_bug.cgi?id=270139
    rdar://123667952

    Reviewed by Pascoe.

    Avoid reading past the end of `m_pendingAsyncCalls`.

    * Source/JavaScriptCore/inspector/agents/InspectorDebuggerAgent.cpp:
    (Inspector::InspectorDebuggerAgent::didScheduleAsyncCall):

    Canonical link: https://commits.webkit.org/272448.684@safari-7618-branch

Source/JavaScriptCore/inspector/agents/InspectorDebuggerAgent.cpp

@@ -444,7 +444,8 @@ void InspectorDebuggerAgent::didScheduleAsyncCall(JSC::JSGlobalObject* globalObj
     if (!m_currentAsyncCallIdentifierStack.isEmpty()) {
         auto it = m_pendingAsyncCalls.find(m_currentAsyncCallIdentifierStack.last());
         ASSERT(it != m_pendingAsyncCalls.end());
-        parentStackTrace = it->value;
+        if (LIKELY(it != m_pendingAsyncCalls.end()))
+            parentStackTrace = it->value;
     }
 
     auto identifier = asyncCallIdentifier(asyncCallType, callbackId);