Fix double free.

This double free involves damaging GHashTable internals (key and value).

Change-Id: I17c8a473b96ab28e611ccf79ddd2b3da56512184
Reviewed-on: https://code.wireshark.org/review/4629
Tested-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Tomasz Moń <desowin@gmail.com>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>