DECL_SOURCE_FILE can be NULL (for example when DECL_ARTIFICIAL).
So make sure to check before comparing against main_input_filename.

gcc/ChangeLog

	PR c/69911
	* cgraphunit.c (check_global_declaration): Check main_input_filename
	and DECL_SOURCE_FILE are not NULL.

Co-Authored-By: Jakub Jelinek <jakub@redhat.com>

From-SVN: r233627