Lua: getting fieldinfo.value for FT_NONE causes assert

Retrieving an FT_NONE field's value through `fieldinfo.value` or
`fieldinfo()` causes an assert.  It should retrieve the label instead.

Bug: 10815
Change-Id: Ia80443f3dd046b2cc31d1dff70d599e96d6f6fe3
Reviewed-on: https://code.wireshark.org/review/6150
Petri-Dish: Hadriel Kaplan <hadrielk@yahoo.com>
Reviewed-by: Michael Mann <mmann78@netscape.net>
Reviewed-by: Hadriel Kaplan <hadrielk@yahoo.com>
Tested-by: Hadriel Kaplan <hadrielk@yahoo.com>