Fix assertion when envval of proc is Qundef

The following code crashes with assertions enabled because envval could
be Qundef:

    {}.to_proc.dup

test/ruby/test_proc.rb

@@ -377,6 +377,7 @@ class TestProc < Test::Unit::TestCase
   end
 
   def test_dup_clone
+    # iseq backed proc
     b = proc {|x| x + "bar" }
     class << b; attr_accessor :foo; end
 
@@ -389,6 +390,24 @@ class TestProc < Test::Unit::TestCase
     assert_equal("foobar", bc.call("foo"))
     bc.foo = :foo
     assert_equal(:foo, bc.foo)
+
+    # ifunc backed proc
+    b = {foo: "bar"}.to_proc
+
+    bd = b.dup
+    assert_equal("bar", bd.call(:foo))
+
+    bc = b.clone
+    assert_equal("bar", bc.call(:foo))
+
+    # symbol backed proc
+    b = :to_s.to_proc
+
+    bd = b.dup
+    assert_equal("testing", bd.call(:testing))
+
+    bc = b.clone
+    assert_equal("testing", bc.call(:testing))
   end
 
   def test_dup_subclass

vm_core.h

@@ -1493,7 +1493,7 @@ VM_ENV_ESCAPED_P(const VALUE *ep)
 static inline int
 vm_assert_env(VALUE obj)
 {
-    VM_ASSERT(imemo_type_p(obj, imemo_env));
+    VM_ASSERT(obj == Qundef || imemo_type_p(obj, imemo_env));
     return 1;
 }
 #endif