[libxml] Fix dict corruption caused by entity reference cycles - #4990

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!

(cherry picked from commit 1b41ec4e9433b05bb0376be4725804c54ef1d80b)