Ensure files cannot be closed multiple times

Also fix an NPE if we try to close them twice.

Fixes #230

src/main/java/dan200/computercraft/core/apis/handles/BinaryReadableHandle.java

@@ -212,6 +212,7 @@ public class BinaryReadableHandle extends HandleGeneric
                 }
             }
             case 3: // close
+                checkOpen();
                 close();
                 return null;
             case 4: // seek

src/main/java/dan200/computercraft/core/apis/handles/BinaryWritableHandle.java

@@ -95,6 +95,7 @@ public class BinaryWritableHandle extends HandleGeneric
                     return null;
                 }
             case 2: // close
+                checkOpen();
                 close();
                 return null;
             case 3: // seek

src/main/java/dan200/computercraft/core/apis/handles/EncodedReadableHandle.java

@@ -152,6 +152,7 @@ public class EncodedReadableHandle extends HandleGeneric
                     return null;
                 }
             case 3: // close
+                checkOpen();
                 close();
                 return null;
             default:

src/main/java/dan200/computercraft/core/apis/handles/EncodedWritableHandle.java

@@ -93,6 +93,7 @@ public class EncodedWritableHandle extends HandleGeneric
                     return null;
                 }
             case 3: // close
+                checkOpen();
                 close();
                 return null;
             default:

src/main/java/dan200/computercraft/core/apis/handles/HandleGeneric.java

@@ -37,8 +37,13 @@ public abstract class HandleGeneric implements ILuaObject
     protected final void close()
     {
         m_open = false;
-        IoUtil.closeQuietly( m_closable );
-        m_closable = null;
+
+        Closeable closeable = m_closable;
+        if( closeable != null )
+        {
+            IoUtil.closeQuietly( closeable );
+            m_closable = null;
+        }
     }
 
     /**