remove recursion from NodePathIterator::isValid()

[lsmith77]

this can have nasty side effects when dealing with larger result sets

Fatal error: Maximum function nesting level of '200' reached, aborting! in /Users/lsmith/htdocs/phpcr_benchmark/vendor/jackalope/jackalope/src/Jackalope/NodePathIterator.php on line 100

Call Stack:
    0.0004     278888   1. {main}() /Users/lsmith/htdocs/phpcr_benchmark/index.php:0
   10.2733   25832696   2. Jackalope\NodePathIterator->valid() /Users/lsmith/htdocs/phpcr_benchmark/index.php:34
   10.2778   25759864   3. Jackalope\NodePathIterator->valid() /Users/lsmith/htdocs/phpcr_benchmark/vendor/jackalope/jackalope/src/Jackalope/NodePathIterator.php:107
   10.2778   25759864   4. Jackalope\NodePathIterator->valid() /Users/lsmith/htdocs/phpcr_benchmark/vendor/jackalope/jackalope/src/Jackalope/NodePathIterator.php:107
   10.2778   25759864   5. Jackalope\NodePathIterator->valid() /Users/lsmith/htdocs/phpcr_benchmark/vendor/jackalope/jackalope/src/Jackalope/NodePathIterator.php:107
...