7145358: SA throws ClassCastException for partially loaded ConstantPool

Summary: In printValueOn() in ConstantPool.java check if the poolHolder is a valid Klass and only then print it.
Reviewed-by: sla, sspitsyn
Contributed-by: NKrystal Mok <sajia@taobao.com>

agent/src/share/classes/sun/jvm/hotspot/oops/ConstantPool.java

@@ -648,7 +648,12 @@ public class ConstantPool extends Oop implements ClassConstants {
   }
 
   public void printValueOn(PrintStream tty) {
-    tty.print("ConstantPool for " + getPoolHolder().getName().asString());
+    Oop holder = poolHolder.getValue(this);
+    if (holder instanceof Klass) {
+      tty.print("ConstantPool for " + ((Klass)holder).getName().asString());
+    } else {
+      tty.print("ConstantPool for partially loaded class");
+    }
   }
 
   public long getObjectSize() {