Better handle broken types when loading default value.

Better handle broken types when loading default value.

	* class.c (mono_class_get_field_token): Don't bail out on broken types
	if the fields array loaded fine.

	* class.c (mono_class_get_field_default_value): Return NULL instead of
	asserting if the constant row lookup fails.

	* icall.c (ves_icall_MonoField_GetRawConstantValue): Raise an exception
	if the default value lookup fails.

	Fixes #693937