Fix comparison of nodesets to strings

Fix two bugs in xmlXPathNodeValHash which could lead to errors when
comparing nodesets to strings:

- Only use contents of text nodes to compute the hash for element nodes.
  Comments, PIs, and other node types don't affect the string-value and
  must be ignored.
- Reset `string` to NULL for node types other than text.

Reported by Aleksei on the mailing list:

    https://mail.gnome.org/archives/xml/2017-September/msg00016.html

result/XPath/tests/strbase

@@ -0,0 +1,13 @@
+
+========================
+Expression: //p[.='abc']
+Object is a Node Set :
+Set contains 2 nodes:
+1  ELEMENT p
+    ATTRIBUTE id
+      TEXT
+        content=1
+2  ELEMENT p
+    ATTRIBUTE id
+      TEXT
+        content=2

test/XPath/docs/str

@@ -6,4 +6,6 @@
   <p><p>a span</p>n<p>ing one</p></p>
   <p><p>and an unbal</p><empty/>anced test</p>
   <p>for empty string <seq>123</seq></p>
+  <p id="1">a<b>b</b>c</p>
+  <p id="2"><!--X-->abc</p>
 </chapter>

test/XPath/tests/strbase

@@ -0,0 +1 @@
+//p[.='abc']

xpath.c

@@ -6400,16 +6400,12 @@ xmlXPathNodeValHash(xmlNodePtr node) {
     }
     while (tmp != NULL) {
 	switch (tmp->type) {
-	    case XML_COMMENT_NODE:
-	    case XML_PI_NODE:
 	    case XML_CDATA_SECTION_NODE:
 	    case XML_TEXT_NODE:
 		string = tmp->content;
 		break;
-	    case XML_NAMESPACE_DECL:
-		string = ((xmlNsPtr)tmp)->href;
-		break;
 	    default:
+                string = NULL;
 		break;
 	}
 	if ((string != NULL) && (string[0] != 0)) {