Fix XPointer paths beginning with range-to

The old code would invoke the broken xmlXPtrRangeToFunction. range-to
isn't really a function but a special kind of location step. Remove
this function and always handle range-to in the XPath code.

The old xmlXPtrRangeToFunction could also be abused to trigger a
use-after-free error with the potential for remote code execution.

Found with afl-fuzz.

Fixes CVE-2016-5131.