kconfig: fix return value of do_error_if()

$(error-if,...) is expanded to an empty string. Currently, it relies on
eval_clause() returning xstrdup("") when all attempts for expansion fail,
but the correct implementation is to make do_error_if() return xstrdup("").

Fixes: 1d6272e6

 ("kconfig: add 'info', 'warning-if', and 'error-if' built-in functions")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>