Fix a double free in the List functions

The code was set up so that it would free the individual items and the data
in `freeListData`, but there was already a Go `defer` to free the data item,
resulting in a double free.

Remove the `free` in `freeListData` and leave the original one.

In addition, move the `defer` for freeing the list data before the error
check, so that the data is also free in the error case. This just removes
a minor leak.

This vulnerability was discovered by:
Jasiel Spelman of Trend Micro Zero Day Initiative and Trend Micro Team Nebula

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

secretservice/secretservice_linux.c

@@ -158,5 +158,4 @@ void freeListData(char *** data, unsigned int length) {
 	for(i=0; i<length; i++) {
 		free((*data)[i]);
 	}
-	free(*data);
 }