Fortran: Fix handling of parsed format strings.

Fortran: Fix handling of parsed format strings.

	Previously parsed strings with errors were being cached such
	that subsequent use of the format string were not being
	checked for errors.

	PR libfortran/119856

libgfortran/ChangeLog:

	* io/format.c (parse_format_list): Set the fmt->error
	message for missing comma.
	(parse_format): Do not cache the parsed format string
	if a previous error ocurred.

gcc/testsuite/ChangeLog:

	* gfortran.dg/pr119856.f90: New test.

(cherry picked from commit 5ff48aabf76c8913c013f233d3f42bb217a16e7b)