Check for null selectors when looping in findBestRuleSetAndAdd

Source/core/css/RuleSet.cpp

Index: Source/core/css/RuleSet.cpp
diff --git a/Source/core/css/RuleSet.cpp b/Source/core/css/RuleSet.cpp
index fefc1e4310b1aa9006b8ffcfa65737dd361a0a0b..a745456129f99c6f77be7d50a853fdfa68f7c5b4 100644
--- a/Source/core/css/RuleSet.cpp
+++ b/Source/core/css/RuleSet.cpp
@@ -177,10 +177,12 @@ bool RuleSet::findBestRuleSetAndAdd(const CSSSelector& component, RuleData& rule
 #endif
 
     const CSSSelector* it = &component;
-    for (; it->relation() == CSSSelector::SubSelector; it = it->tagHistory()) {
+    for (; it && it->relation() == CSSSelector::SubSelector; it = it->tagHistory()) {
         extractValuesforSelector(it, id, className, customPseudoElementName, tagName);
     }
-    extractValuesforSelector(it, id, className, customPseudoElementName, tagName);
+    // FIXME: this null check should not be necessary. See crbug.com/358475
+    if (it)
+        extractValuesforSelector(it, id, className, customPseudoElementName, tagName);
 
     // Prefer rule sets in order of most likely to apply infrequently.
     if (!id.isEmpty()) {