Initialise _dataptr in constructor.

Fix a bug found by a unittest.

arangod/V8Server/v8-query.cpp

@@ -1775,7 +1775,6 @@ static v8::Handle<v8::Value> JS_AnyQuery (v8::Arguments const& argv) {
   }
 
   TRI_doc_mptr_copy_t document;
-  document.setDataPtr(nullptr);  // PROTECTED by stack locality
 
   V8ReadTransaction trx(col->_vocbase, col->_cid);
 

arangod/VocBase/document-collection.h

@@ -117,8 +117,8 @@ struct TRI_doc_mptr_t {
     TRI_doc_mptr_t*        _prev;    // previous master pointer
     TRI_doc_mptr_t*        _next;    // next master pointer
 
-    TRI_doc_mptr_t () : _rid(0), _fid(0), _hash(0), _prev(nullptr), 
-                        _next(nullptr) {
+    TRI_doc_mptr_t () : _rid(0), _fid(0), _dataptr(nullptr), _hash(0), 
+                        _prev(nullptr), _next(nullptr) {
     }
 
     void clear () {