Fixes a hang on an invalid ID in a WAIT statement.

gcc/fortran/ChangeLog:

2020-05-23  Thomas Koenig  <tkoenig@gcc.gnu.org>

	PR libfortran/95191
	* libgfortran.h (libgfortran_error_codes): Add
	LIBERROR_BAD_WAIT_ID.

libgfortran/ChangeLog:

2020-05-23  Thomas Koenig  <tkoenig@gcc.gnu.org>

	PR libfortran/95191
	* io/async.c (async_wait_id): Generate error if ID is higher
	than the highest current ID.
	* runtime/error.c (translate_error): Handle LIBERROR_BAD_WAIT_ID.

libgomp/ChangeLog:

2020-05-23  Thomas Koenig  <tkoenig@gcc.gnu.org>

	PR libfortran/95191
	* testsuite/libgomp.fortran/async_io_9.f90: New test.