Harden xfce_gtk_handle_tab_accels (thunar#1067)

In order to prevent segfault on malicious input:

• g_error will exit the application ... better to just warn and use the first accel found
• gtk_accel_group_query will not fill group_entries_count on internal errors, so it needs to be initialized
• Directly return if the passed accel_group is not an accel group (instead of waiting until gtk_accel_group_query does so)

(The actual cause of thunar#1067 (closed) seems to be that thunar does not pass a valid accel group, though as well this change already will prevent the crash)

@Tamaranch, fine for you like this ?