8292368: [ppc64] internal error g1BarrierSet.inline.hpp assert(oopDesc::is_oop(pre_val, true)) failed: Error

Reviewed-by: eosterlund, shade

src/hotspot/share/gc/shared/barrierSetNMethod.cpp

@@ -72,8 +72,10 @@ bool BarrierSetNMethod::nmethod_entry_barrier(nmethod* nm) {
       // conversion that performs the load barriers. This is too subtle, so we instead
       // perform an explicit keep alive call.
       oop obj = NativeAccess<ON_PHANTOM_OOP_REF | AS_NO_KEEPALIVE>::oop_load(p);
+      if (obj != nullptr) {
         Universe::heap()->keep_alive(obj);
       }
+    }
 
     virtual void do_oop(narrowOop* p) { ShouldNotReachHere(); }
   };