Do not iterate past the end on processing leaf class.

tools/clang/blink_gc_plugin/tests/left_most_gc_base.txt

Index: tools/clang/blink_gc_plugin/tests/left_most_gc_base.txt
diff --git a/tools/clang/blink_gc_plugin/tests/left_most_gc_base.txt b/tools/clang/blink_gc_plugin/tests/left_most_gc_base.txt
index 4766ea67dec0cda85cf5d359ee64035013426fce..e2d04186773d828a5a180fc250f60d92a983b6a9 100644
--- a/tools/clang/blink_gc_plugin/tests/left_most_gc_base.txt
+++ b/tools/clang/blink_gc_plugin/tests/left_most_gc_base.txt
@@ -5,4 +5,10 @@ class Right : public A, public B, public GarbageCollected<Right> { };  // Error
 ./left_most_gc_base.h:18:1: warning: [blink-gc] Class 'DerivedRight' must derive its GC base in the left-most position.
 class DerivedRight : public Right, public Left { };  // Error
 ^
-2 warnings generated.
+./left_most_gc_base.h:12:1: warning: [blink-gc] Left-most base class 'A' of derived class 'IllFormed' must be polymorphic.
+class A { };
+^
+./left_most_gc_base.h:26:1: warning: [blink-gc] Class 'IllFormed' must derive its GC base in the left-most position.
+class IllFormed : public A, public C { }; // Error
+^
+4 warnings generated.