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.
diff --git a/xpath.c b/xpath.c
index ff2f2da..ce46c49 100644
--- a/xpath.c
+++ b/xpath.c
@@ -736,6 +736,7 @@
 	comp->steps = real;
     }
     comp->last = comp->nbStep;
+    comp->steps[comp->nbStep].rewriteType = 0;
     comp->steps[comp->nbStep].ch1 = ch1;
     comp->steps[comp->nbStep].ch2 = ch2;
     comp->steps[comp->nbStep].op = op;