[CVE-2022-40304] Fix dict corruption caused by entity reference cycles

[CVE-2022-40304] Fix dict corruption caused by entity reference cycles

When an entity reference cycle is detected, the entity content is
cleared by setting its first byte to zero. But the entity content might
be allocated from a dict. In this case, the dict entry becomes corrupted
leading to all kinds of logic errors, including memory errors like
double-frees.

Stop storing entity content, orig, ExternalID and SystemID in a dict.
These values are unlikely to occur multiple times in a document, so they
shouldn't have been stored in a dict in the first place.

Thanks to Ned Williamson and Nathan Wachholz working with Google Project
Zero for the report!

Change-Id: Ic72980e9951365347f484203817b37fa41e65d52
Signed-off-by: DongHun Kwak <dh0128.kwak@samsung.com>