menu: fix crash when opening SDCard menu while printing

This fixes the bug introduced by f1091a4.
The problem is that condition was returning string which
could not be converted to boolean. We solve that by showing
`Start Printing` when `is_active`.

```
ERROR:root:Unhandled exception during run
Traceback (most recent call last):
   ...
   File "/opt/klipper/klippy/extras/display/menu.py", line 117, in eval_enable
     return bool(ast.literal_eval(self._enable_tpl.render(context)))
   File "/usr/lib/python2.7/ast.py", line 49, in literal_eval
     node_or_string = parse(node_or_string, mode='eval')
   File "/usr/lib/python2.7/ast.py", line 37, in parse
     return compile(source, filename, mode, PyCF_ONLY_AST)
   File "<unknown>", line 1
     /opt/octoprint/uploads/wait_print.gcode
```

Signed-off-by: Kamil Trzcinski <[email protected]>

klippy/extras/display/menu.cfg

@@ -165,7 +165,7 @@ name: SD Card
 
 [menu __main __sdcard __start]
 type: command
-enable: {('virtual_sdcard' in printer) and printer.virtual_sdcard.file_path}
+enable: {('virtual_sdcard' in printer) and printer.virtual_sdcard.file_path and not printer.virtual_sdcard.is_active}
 name: Start printing
 gcode: M24