Implement buffer overrun check for temporary that holds mangled names.

2020-05-30  Harald Anlauf  <anlauf@gmx.de>

gcc/fortran/
	PR fortran/95090
	* class.c (get_unique_type_string): Use buffer overrun check.

(cherry picked from commit bf5fbbbd8c9a3385c1083cc80683bdb0195b1ffc)