Fixed an bug in xmlXPathCompExprAdd(): the newly introduced field

* xpath.c: Fixed an bug in xmlXPathCompExprAdd(): the newly
  introduced field @rewriteType on xmlXPathStepOp was not
  initialized to zero here; this could lead to the activation
  of the axis rewrite code in xmlXPathNodeCollectAndTest() when
  @rewriteType is randomly set to the value 1. A test
  (hardcoding the intial value to 1) revealed that the
  resulting incorrect behaviour is similar to the behaviour
  as described by Arnold Hendriks on the mailing list; so I
  hope that will fix the issue.