Refactoring code so as to avoid possible crash due to null pointer

Source/core/editing/InputMethodController.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 300 matching lines @@
             frame().selection().setSelectedRange(selectedRange.get(), DOWNSTREAM, FrameSelection::NonDirectional, NotUserTriggered);
         }
     }
 }
 
 void InputMethodController::setCompositionFromExistingText(const Vector<CompositionUnderline>& underlines, unsigned compositionStart, unsigned compositionEnd)
 {
     Element* editable = frame().selection().rootEditableElement();
     Position base = frame().selection().base().downstream();
     Node* baseNode = base.anchorNode();
-    if (editable->firstChild() == baseNode && editable->lastChild() == baseNode && baseNode->isTextNode()) {
+    if (baseNode && editable->firstChild() == baseNode && editable->lastChild() == baseNode && baseNode->isTextNode()) {
         m_compositionNode = nullptr;
         m_customCompositionUnderlines.clear();
 
         if (base.anchorType() != Position::PositionIsOffsetInAnchor)
             return;
-        if (!baseNode || baseNode != frame().selection().extent().anchorNode())
+        if (baseNode != frame().selection().extent().anchorNode())
             return;
 
         m_compositionNode = toText(baseNode);
         RefPtrWillBeRawPtr<Range> range = PlainTextRange(compositionStart, compositionEnd).createRange(*editable);
         if (!range)
             return;
 
         m_compositionStart = range->startOffset();
         m_compositionEnd = range->endOffset();
         m_customCompositionUnderlines = underlines;
@@ skipping 87 matching lines @@
     TypingCommand::deleteSelection(*frame().document());
 }
 
 void InputMethodController::trace(Visitor* visitor)
 {
     visitor->trace(m_frame);
     visitor->trace(m_compositionNode);
 }
 
 } // namespace blink