Do not iterate past the end on processing leaf class.

tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp

 // Copyright 2014 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.
 
 // This clang plugin checks various invariants of the Blink garbage
 // collection infrastructure.
 //
 // Errors are described at:
 // http://www.chromium.org/developers/blink-gc-plugin-errors
 
@@ skipping 1082 matching lines @@
             return;
         }
         ReportBaseClassMustDeclareVirtualTrace(info, left_most);
         return;
       }
 
       // Check condition (2):
       if (DeclaresVirtualMethods(left_most))
         return;
       if (left_most_base) {
-        ++it; // Get the base next to the "safe polymorphic base"
+        // Get the base next to the "safe polymorphic base"
+        if (it != left_most->bases_end())
+          ++it;
         if (it != left_most->bases_end()) {
           if (CXXRecordDecl* next_base = it->getType()->getAsCXXRecordDecl()) {
             if (CXXRecordDecl* next_left_most = GetLeftMostBase(next_base)) {
               if (DeclaresVirtualMethods(next_left_most))
                 return;
               ReportLeftMostBaseMustBePolymorphic(info, next_left_most);
               return;
             }
           }
         }
@@ skipping 792 matching lines @@
 
  private:
   BlinkGCPluginOptions options_;
 };
 
 }  // namespace
 
 static FrontendPluginRegistry::Add<BlinkGCPluginAction> X(
     "blink-gc-plugin",
     "Check Blink GC invariants");