lto/94822 - fix ICE in component_ref_size

This ICE appears because gcc will stream it to the function_body section
when processing the variable with the initial value of the constructor
type, and the error_mark_node to the decls section.
When recompiling, the value obtained with DECL_INITIAL will be error_mark.

2020-04-29  Richard Biener  <rguenther@suse.de>
	    Li Zekun  <lizekun1@huawei.com>

	PR lto/94822
	* tree.c (component_ref_size): Guard against error_mark_node
	DECL_INITIAL as it happens with LTO.

	* gcc.dg/lto/pr94822_0.c: New testcase.
	* gcc.dg/lto/pr94822_1.c: Alternate file.
	* gcc.dg/lto/pr94822.h: Likewise.