Fortran: Fix runtime segfault closing negative unit

Fortran: Fix runtime segfault closing negative unit

	When closing a UNIT with an invalid negative unit
	number, a segfault ensued. This patch adds checks
	for these conditions and issues errors.

	PR libfortran/119502

libgfortran/ChangeLog:

	* io/close.c (st_close): Issue an error and avoid
	calling close_share when there is no stream assigned.
	* io/open.c (st_open): If there is no stream assigned
	to the unit, unlock the unit and issue an error.

gcc/testsuite/ChangeLog:

	* gfortran.dg/pr119502.f90: New test.