Referencing a unit in a WAIT statement that has not been opened before
resulted in a NULL pointer dereference.  Check for this condition.

2020-05-26  Harald Anlauf  <anlauf@gmx.de>

libgfortran/
	PR libfortran/95104
	* io/transfer.c (st_wait_async): Do not dereference NULL pointer.

gcc/testsuite/
	PR libfortran/95104
	* gfortran.dg/pr95104.f90: New test.

Co-Authored-By: Steven G. Kargl  <kargl@gcc.gnu.org>
(cherry picked from commit 56f03cd12be26828788a27f6f3c250041a958e45)