Reject unsupported external iloc data references

[jmestwa-coder]

libavif currently supports same-file item extents and idat extents only.

iloc.data_reference_index values were previously parsed but ignored, causing extents declared as external references to be interpreted as same-file offsets.

This change rejects unsupported nonzero data_reference_index values during iloc parsing so downstream extent handling operates under validated source-boundary assumptions.

Changes

• Reject nonzero iloc.data_reference_index values in avifParseItemLocationBox()
• Add a regression test covering unsupported external data references

Test

• Mutate white_1x1.avif to set data_reference_index = 1
• Verify parsing now fails with AVIF_RESULT_BMFF_PARSE_FAILED
• Existing IlocTest.TwoExtents continues to pass

[wantehchang]

Thank you for the pull request. ISO/IEC 14496-12:2022 Clause 8.11.3 (Item location box) does not require or recommend that data_reference_index be set to 0 when this field is not used. Therefore I think it is better to simply ignore this field.

Note that the value 0 is a valid value for data_reference_index, so one can argue that the value 0 can't really be used to indicate that the field is not used.

[wantehchang]

I took a closer look. I think the condition at line 2057 should also check that construction_method is equal to 0:

        if (constructionMethod == 0 && dataReferenceIndex != 0) {

But it is not clear what value constructionMethod should be set to for version 0. Do you know?

[jmestwa-coder]

Thanks for taking a closer look.

From what I understand, construction_method is only explicitly present starting from iloc version 1/2, so for version 0 I’m not fully sure what the intended interpretation should be either.

I’ve updated the test to make the fixture assumptions explicit before mutating data_reference_index.

[wantehchang]

I suggest also verifying data_reference_index is equal to 0 before changing it to 1:

   ASSERT_EQ(iloc_position[4], 0);
+  ASSERT_EQ(iloc_position[14], 0);
+  ASSERT_EQ(iloc_position[15], 0);
   iloc_position[14] = 0;
   iloc_position[15] = 1;

[jmestwa-coder]

I verified the fixture layout and added assertions checking the original data_reference_index value before mutating it.