Set a maximum limit on element recursion depth

The maximum recursion depth is set to 25. If that's not sufficient for
someone's needs, they're welcome to submit a patch that makes it

This fixes a stack overflow found by AutoFuzz. The stack overflow was
caused by recursing too much. I've included the fuzzer's test cases in
the fuzzing/corpus directory. I've also created two (one for SimpleTag
and ChapterAtom each) additional test files, each comprised of 10,000
recursively nested elements. Manually running the fuzzer (with ASan and
UBSan) shows all inputs are now handled correctly, and no stack
overflows occur.

Change-Id: I8514259fd0788e71a58e3ccce2a0fb8a3523acfc
27 files changed