Harden changing font style over unknown elements.

Source/core/editing/ApplyStyleCommand.cpp

Index: Source/core/editing/ApplyStyleCommand.cpp
diff --git a/Source/core/editing/ApplyStyleCommand.cpp b/Source/core/editing/ApplyStyleCommand.cpp
index f141674f0dccd60e523740a9a121876654c30416..2da7751ff24cbc956d28ed4abef81f559e37868d 100644
--- a/Source/core/editing/ApplyStyleCommand.cpp
+++ b/Source/core/editing/ApplyStyleCommand.cpp
@@ -372,8 +372,12 @@ void ApplyStyleCommand::applyRelativeFontStyleChange(EditingStyle* style)
     if (startNode == beyondEnd)
         return;
 
-    if (startNode->isTextNode() && start.deprecatedEditingOffset() >= caretMaxOffset(startNode)) // Move out of text node if range does not include its characters.
+    if (startNode->isTextNode() && start.deprecatedEditingOffset() >= caretMaxOffset(startNode)) {
+        // Move out of text node if range does not include its characters.
         startNode = NodeTraversal::next(*startNode);
+        if (!startNode)
+            return;
+    }
 
     // Store away font size before making any changes to the document.
     // This ensures that changes to one node won't effect another.