Fix NULL pointer deref in XPointer range-to

- Check for errors after evaluating first operand.
- Add sanity check for empty stack.

Found with afl-fuzz.

result/XPath/xptr/viderror

@@ -0,0 +1,4 @@
+
+========================
+Expression: xpointer(non-existing-fn()/range-to(id('chapter2')))
+Object is empty (NULL)

test/XPath/xptr/viderror

@@ -0,0 +1 @@
+xpointer(non-existing-fn()/range-to(id('chapter2')))

xpath.c

@@ -14005,9 +14005,14 @@ xmlXPathCompOpEval(xmlXPathParserContextPtr ctxt, xmlXPathStepOpPtr op)
                 xmlNodeSetPtr oldset;
                 int i, j;
 
-                if (op->ch1 != -1)
+                if (op->ch1 != -1) {
                     total +=
                         xmlXPathCompOpEval(ctxt, &comp->steps[op->ch1]);
+                    CHECK_ERROR0;
+                }
+                if (ctxt->value == NULL) {
+                    XP_ERROR0(XPATH_INVALID_OPERAND);
+                }
                 if (op->ch2 == -1)
                     return (total);