Fix crash when uninstalling snap

Issue:

app_data is an argument to both the actual and progress callbacks of snapd_client_remove2_async (). In this case, using g_steal_pointer on one of them will render the other one to be NULL, causing the crash.

Fix:

Follow the same convention used in install operation.

Additional details:

(gdb) bt
#0  0x00007fdb54b8f215 in uninstall_progress_cb (client=0x55b254688280 [SnapdClient], change=0x55b2506448a0 [SnapdChange], deprecated=0x55b253357980, user_data=0x0) at ../plugins/snap/gs-plugin-snap.c:2375
#1  0x00007fdb54b1a552 in update_changes (self=0x55b254688280 [SnapdClient], change=0x55b2506448a0 [SnapdChange], data=0x55b254c1c150) at ../snapd-glib/snapd-client.c:450
l#2  0x00007fdb54b1b621 in parse_response (body=0x55b252cc64e0, content_type=<optimized out>, status_code=<optimized out>, request=0x55b2539206f0 [SnapdGetChange], self=0x55b254688280 [SnapdClient])
    at ../snapd-glib/snapd-client.c:504
#3  read_cb (socket=<optimized out>, condition=<optimized out>, self=0x55b254688280 [SnapdClient]) at ../snapd-glib/snapd-client.c:696
#4  0x00007fdb664a58db in socket_source_dispatch (source=0x55b254c8e730, callback=0x7fdb54b1b2e0 <read_cb>, user_data=0x55b254688280) at ../../../gio/gsocket.c:4267
#5  0x00007fdb662a67df in g_main_dispatch (context=context@entry=0x55b24d30b6f0) at ../../../glib/gmain.c:3357
#6  0x00007fdb662a8a17 in g_main_context_dispatch_unlocked (context=0x55b24d30b6f0) at ../../../glib/gmain.c:4208
i#7  g_main_context_iterate_unlocked (context=context@entry=0x55b24d30b6f0, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at ../../../glib/gmain.c:4273
#8  0x00007fdb662a9180 in g_main_context_iteration (context=context@entry=0x55b24d30b6f0, may_block=may_block@entry=1) at ../../../glib/gmain.c:4338
#9  0x00007fdb664e6305 in g_application_run (application=application@entry=0x55b24d2d26c0 [GsApplication], argc=argc@entry=2, argv=argv@entry=0x7ffde81ea458) at ../../../gio/gapplication.c:2715
#10 0x000055b24039f5bf in main (argc=2, argv=0x7ffde81ea458) at ../src/gs-main.c:49

(gdb) list
2370	                       SnapdChange *change,
2371	                       gpointer     deprecated,
2372	                       gpointer     user_data)
2373	{
2374		UninstallSingleAppData *app_data = user_data;
2375		GTask *task = app_data->task;
2376		GsPluginSnap *self = g_task_get_source_object (task);
2377		UninstallAppsData *data = g_task_get_task_data (task);
2378		GPtrArray *tasks;
2379		gint64 done = 0, total = 0;

(gdb) p app_data
$3 = (UninstallSingleAppData *) 0x0
(gdb)