DERBY-6626: Check type of user-supplied modules before creating instances

Check that the encryptionProvider connection attribute specifies an
implementation of java.security.Provider before trying to create an
instance of it.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1607266 13f79535-47bb-0310-9956-ffa450edef68