6898310: (cs) Charset cache lookups should be synchronized

Summary: synchronize the lookup in iterator
Reviewed-by: alanb

src/share/classes/sun/nio/cs/AbstractCharsetProvider.java

@@ -179,8 +179,10 @@ public class AbstractCharsetProvider
 
                 public Charset next() {
                     String csn = i.next();
+                    synchronized (AbstractCharsetProvider.this) {
                         return lookup(csn);
                     }
+                }
 
                 public void remove() {
                     throw new UnsupportedOperationException();