Properly fall back if can't setAccessible

Changes in e8dbe9c broke this fallback by silencing errors from
the old setAccessible (as part of becoming more module-friendly).
This patch checks if the setAccessible was successful and returns
null for any fields that could not be made accessible. This in
turn allows the USABLE fallback to properly reflect actual
usability.

core/src/main/java/org/jruby/util/io/SeekableByteChannelImpl.java

@@ -149,15 +149,17 @@ private static int readIntField(ByteArrayInputStream self, Field field) {
     private static Field accessibleField(final String name) {
         try {
             Field field = ByteArrayInputStream.class.getDeclaredField(name);
-            Java.trySetAccessible(field);
-            return field;
+            if (Java.trySetAccessible(field)) {
+                return field;
+            }
         }
         catch (NoSuchFieldException ex) {
-            return null; // should never happen
+            // should never happen
         }
         catch (SecurityException ex) {
-            return null;
         }
+
+        return null;
     }
 
 }