HotSpot must check attribute_length of EnclosingMethod attribute

During class loading, HotSpot must check that an EnclosingMethod attribute is of the proper length. Namely, its attribute_length item must be four.

Similar checks are already made on other attributes, even those not used directly by the JVM. For example, HotSpot checks that the attribute_length of Synthetic, Signature, and SourceFile is zero, two, and two respectively. As of JDK 8u5, EnclosingMethod is the only attribute with a checkable attribute_length that is not checked by HotSpot.