[Bug #20322] Fix rb_enc_interned_str_cstr null encoding

The documentation for `rb_enc_interned_str_cstr` notes that `enc` can be
a null pointer, but this currently causes a segmentation fault when
trying to autoload the encoding. This commit fixes the issue by checking
for NULL before calling `rb_enc_autoload`.

ext/-test-/string/fstring.c

@@ -19,13 +19,13 @@ bug_s_fstring_fake_str(VALUE self)
 VALUE
 bug_s_rb_enc_interned_str(VALUE self, VALUE encoding)
 {
-    return rb_enc_interned_str("foo", 3, RDATA(encoding)->data);
+    return rb_enc_interned_str("foo", 3, NIL_P(encoding) ? NULL : RDATA(encoding)->data);
 }
 
 VALUE
 bug_s_rb_enc_str_new(VALUE self, VALUE encoding)
 {
-    return rb_enc_str_new("foo", 3, RDATA(encoding)->data);
+    return rb_enc_str_new("foo", 3, NIL_P(encoding) ? NULL : RDATA(encoding)->data);
 }
 
 void

spec/ruby/optional/capi/ext/string_spec.c

@@ -573,7 +573,7 @@ static VALUE string_spec_rb_str_unlocktmp(VALUE self, VALUE str) {
 }
 
 static VALUE string_spec_rb_enc_interned_str_cstr(VALUE self, VALUE str, VALUE enc) {
-  rb_encoding *e = rb_to_encoding(enc);
+  rb_encoding *e = NIL_P(enc) ? 0 : rb_to_encoding(enc);
   return rb_enc_interned_str_cstr(RSTRING_PTR(str), e);
 }
 

spec/ruby/optional/capi/string_spec.rb

@@ -1236,6 +1236,14 @@ def inspect
     it "returns the same string as String#-@" do
       @s.rb_enc_interned_str_cstr("hello", Encoding::UTF_8).should.equal?(-"hello")
     end
+
+    ruby_bug "#20322", ""..."3.4" do
+      it "uses the default encoding if encoding is null" do
+        str = "hello"
+        val = @s.rb_enc_interned_str_cstr(str, nil)
+        val.encoding.should == Encoding::ASCII_8BIT
+      end
+    end
   end
 
   describe "rb_str_to_interned_str" do

string.c

@@ -12122,7 +12122,7 @@ rb_interned_str_cstr(const char *ptr)
 VALUE
 rb_enc_interned_str(const char *ptr, long len, rb_encoding *enc)
 {
-    if (UNLIKELY(rb_enc_autoload_p(enc))) {
+    if (enc != NULL && UNLIKELY(rb_enc_autoload_p(enc))) {
         rb_enc_autoload(enc);
     }
 

test/-ext-/string/test_fstring.rb

@@ -20,6 +20,10 @@ def test_rb_enc_interned_str_autoloaded_encoding
     RUBY
   end
 
+  def test_rb_enc_interned_str_null_encoding
+    assert_equal Encoding::ASCII_8BIT, Bug::String.rb_enc_interned_str(nil).encoding
+  end
+
   def test_rb_enc_str_new_autoloaded_encoding
     assert_separately([], <<~RUBY)
       require '-test-/string'
@@ -28,6 +32,10 @@ def test_rb_enc_str_new_autoloaded_encoding
     RUBY
   end
 
+  def test_rb_enc_str_new_null_encoding
+    assert_equal Encoding::ASCII_8BIT, Bug::String.rb_enc_str_new(nil).encoding
+  end
+
   def test_instance_variable
     str = __method__.to_s * 3
     str.instance_variable_set(:@test, 42)