8336346: Fix -Wzero-as-null-pointer-constant warnings in jvmciJavaClasses.cpp

Reviewed-by: jwaters, thartmann

Author: Kim Barrett

src/hotspot/share/jvmci/jvmciJavaClasses.cpp

@@ -584,7 +584,7 @@ void JNIJVMCI::register_natives(JNIEnv* env) {
 #define EMPTY2(x,y)
 #define FIELD3(className, name, sig) FIELD2(className, name)
 #define FIELD2(className, name) \
-  jfieldID JNIJVMCI::className::_##name##_field_id = 0; \
+  jfieldID JNIJVMCI::className::_##name##_field_id = nullptr; \
   int HotSpotJVMCI::className::_##name##_offset = 0;
 #define METHOD(jniCallType, jniGetMethod, hsCallType, returnType, className, methodName, signatureSymbolName)
 #define CONSTRUCTOR(className, signature)
@@ -626,7 +626,7 @@ JVMCI_CLASSES_DO(EMPTY2, EMPTY0, FIELD2, FIELD2, FIELD2, FIELD2, FIELD2, FIELD3,
   void JNIJVMCI::className::check(JVMCIEnv* jvmciEnv, JVMCIObject obj, const char* field_name, jfieldID offset) {                 \
     assert(obj.is_non_null(), "null field access of %s.%s", #className, field_name);                                              \
     assert(jvmciEnv->isa_##className(obj), "wrong class, " #className " expected, found %s", jvmciEnv->klass_name(obj));          \
-    assert(offset != 0, "must be valid offset");                                                                                  \
+    assert(offset != nullptr, "must be valid offset");                                                                            \
   }                                                                                                                               \
   jclass JNIJVMCI::className::_class = nullptr;