Avoid trigger an assertion failure in the STABS parser by replacing the abort with...

Avoid trigger an assertion failure in the STABS parser by replacing the abort with an error return.

	PR 25714
	* wrstabs.c (stab_pop_type): Replace assertion with error return.
	* write_stabs_in_sections_debugging_info: Likewise.
	* stab_enum_type: Likewise.
	* stab_modify_type: Likewise.
	* stab_struct_field: Likewise.
	* stab_end_struct_type: Likewise.
	* stab_start_class_type: Likewise.
	* stab_class_static_member: Likewise.
	* stab_class_baseclass: Likewise.
	* stab_class_start_method: Likewise.
	* stab_class_method_var: Likewise.
	* stab_class_end_method: Likewise.
	* stab_end_class_type: Likewise.
	* stab_typedef_type: Likewise.
	* stab_start_function: Likewise.
	* stab_end_block: Likewise.
	* stab_lineno: Likewise.