8180575: Null pointer dereference in LoadNode::Identity()

Summary: Fixed a missing null check on the return value of AddPNode::Ideal_base_and_offset() found by Parfait.
Reviewed-by: kvn

src/share/vm/opto/memnode.cpp

@@ -1156,6 +1156,9 @@ Node *LoadNode::Identity( PhaseTransform *phase ) {
       // Use _idx of address base (could be Phi node) for boxed values.
       intptr_t   ignore = 0;
       Node*      base = AddPNode::Ideal_base_and_offset(in(Address), phase, ignore);
+      if (base == NULL) {
+        return this;
+      }
       this_iid = base->_idx;
     }
     const Type* this_type = bottom_type();