gengtype.c (start_root_entry): New function, split out from write_root.

gengtype.c (start_root_entry): New function, split out from write_root.

gcc/
	* gengtype.c (start_root_entry): New function, split out from
	write_root.  Check whether V is null and raise an error if so.
	(write_field_root): Check for V being null.  Don't raise an error here;
	set V to null instead.
	(write_root): Update comment above function.  Use start_root_entry.

From-SVN: r162314