8319961: JvmtiEnvBase doesn't zero _ext_event_callbacks

Backport-of: 97ea5bf0ffafaf8009c19483b9a9b1c30401cf9a

Author: Roman Marchenko

src/hotspot/share/prims/jvmtiEnvBase.cpp

@@ -213,7 +213,8 @@ JvmtiEnvBase::JvmtiEnvBase(jint version) : _env_event_enable() {
   _is_retransformable = true;
 
   // all callbacks initially null
-  memset(&_event_callbacks,0,sizeof(jvmtiEventCallbacks));
+  memset(&_event_callbacks, 0, sizeof(jvmtiEventCallbacks));
+  memset(&_ext_event_callbacks, 0, sizeof(jvmtiExtEventCallbacks));
 
   // all capabilities initially off
   memset(&_current_capabilities, 0, sizeof(_current_capabilities));