PR fortran/96381 - invalid read in gfc_find_derived_vtab

An invalid declaration of a CLASS instance can lead to an internal state
with inconsistent attributes during parsing that needs to be handled with
sufficient care when processing subsequent statements.  Avoid a lookup of
the vtab entry for such cases.

gcc/fortran/ChangeLog:

	* class.c (gfc_find_vtab): Add check on attribute is_class.