evaluate_subexp_standard: Eliminate one goto

evaluate_subexp_standard: Eliminate one goto

A following patch will want to factor out a bit of
evaluate_subexp_standard, and it'd be handy to reuse the code under the
"nosideret:" label there too.  This commits moves it to a separate
function as preparation for that.

gdb/ChangeLog:
2017-09-04  Pedro Alves  <palves@redhat.com>

	* eval.c (eval_skip_value): New function.
	(evaluate_subexp_standard): Use it.