Use RTEST and add test for GH-6832

Technically we shouldn't see Qfalse now, but RTEST also compiles down to
just one branch anyways. Pretty contrived issue, but easy to fix.

struct.c

@@ -48,7 +48,7 @@ struct_ivar_get(VALUE c, ID id)
 
     for (;;) {
         c = rb_class_superclass(c);
-        if (c == 0 || c == rb_cStruct || c == rb_cData || c == Qnil)
+        if (c == rb_cStruct || c == rb_cData || !RTEST(c))
             return Qnil;
         RUBY_ASSERT(RB_TYPE_P(c, T_CLASS));
         ivar = rb_attr_get(c, id);

test/ruby/test_marshal.rb

@@ -92,6 +92,14 @@ class TestMarshal < Test::Unit::TestCase
     TestMarshal.instance_eval { remove_const :StructInvalidMembers }
   end
 
+  def test_load_range_as_struct
+    assert_raise(TypeError, 'GH-6832') do
+      # Can be obtained with:
+      #  $ ruby -e 'Range = Struct.new(:a, :b, :c); p Marshal.dump(Range.new(nil, nil, nil))'
+      Marshal.load("\x04\bS:\nRange\b:\x06a0:\x06b0:\x06c0")
+    end
+  end
+
   class C
     def initialize(str)
       @str = str