[Truffle] Fix FindBugs warning about unsafe cast.

Author: eregon

truffle/src/main/java/org/jruby/truffle/nodes/core/UnboundMethodNodes.java

@@ -76,7 +76,8 @@ private RubyClass metaClass(VirtualFrame frame, Object object) {
         public RubyMethod bind(VirtualFrame frame, RubyUnboundMethod unboundMethod, Object object) {
             notDesignedForCompilation();
             RubyModule module = unboundMethod.getMethod().getDeclaringModule();
-            if (!ModuleOperations.canBindMethodTo(module, metaClass(frame, object))) {
+            // the (redundant) instanceof is to satisfy FindBugs with the following cast
+            if (module instanceof RubyClass && !ModuleOperations.canBindMethodTo(module, metaClass(frame, object))) {
                 CompilerDirectives.transferToInterpreter();
                 if (((RubyClass) module).isSingleton()) {
                     throw new RaiseException(getContext().getCoreLibrary().typeError("singleton method called for a different object", this));