The initial commit for this PR uncovered a latent issue with unit locking
in the Fortran run-time library.  Add check for valid unit.

2020-05-28  Harald Anlauf  <anlauf@gmx.de>

libgfortran/
	PR libfortran/95104
	* io/unit.c (unlock_unit): Guard by check for NULL pointer.

(cherry picked from commit 6ce3d791dfcba469e709935aba5743640f7d4959)