Fix many places where malloc failures aren't reported.
Rework XPath object cache to store free objects in a linked list to
avoid allocating an additional array. Remove some unneeded object pools.
Remove the libfuzzer max_len option which doesn't apply to other
fuzzing engines. Enforce the maximum length directly in the fuzz
targets. For the xml target, lower the maximum when expanding entities
to avoid timeout and OOM errors.
Always limit nested functions calls to 5000. This avoids call stack
overflows with deeply nested expressions.
The expression parser produces about 10 nested function calls when
parsing a subexpression in parentheses, so the effective nesting limit
is about 500 which should be more than enough.
Use a lower limit when fuzzing to account for increased memory usage
when using sanitizers.
