PR fortran/95104 - Segfault on a legal WAIT statement

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)

libgfortran/io/unit.c

@@ -767,9 +767,12 @@ close_unit_1 (gfc_unit *u, int locked)
 void
 unlock_unit (gfc_unit *u)
 {
-  NOTE ("unlock_unit = %d", u->unit_number);
-  UNLOCK (&u->lock);
-  NOTE ("unlock_unit done");
+  if (u)
+    {
+      NOTE ("unlock_unit = %d", u->unit_number);
+      UNLOCK (&u->lock);
+      NOTE ("unlock_unit done");
+    }
 }
 
 /* close_unit()-- Close a unit.  The stream is closed, and any memory