plug-ins: fix cherry-picked commit.

Well I knew that cherry-picking without proper build test was wrong! :P
Commit 657a754f unref() a variable which
does not exist in older GIMP 2.8 code.

plug-ins/common/file-gbr.c

@@ -417,7 +417,7 @@ load_image (const gchar  *filename,
         {
           g_set_error (error, G_FILE_ERROR, G_FILE_ERROR_FAILED,
                        _("Unsupported brush format"));
-          g_object_unref (input);
+          close (fd);
           return -1;
         }
       break;