Don't write to inferior_ptid in corelow.c
    
    gdb/ChangeLog:
    2020-06-18  Pedro Alves  <palves@redhat.com>
    
    	* corelow.c (core_target::close): Use switch_to_no_thread instead
    	of writing to inferior_ptid directly.
    	(add_to_thread_list, core_target_open): Use switch_to_thread
    	instead of writing to inferior_ptid directly.