kconfig: clear expr::val_is_valid when allocated

Since commit 95573cac ("kconfig: cache expression values"), xconfig
emits a lot of false-positive "unmet direct dependencies" warnings.

While conf_read() clears val_is_valid flags, 'make xconfig' calculates
symbol values even before the conf_read() call. This is another issue
that should be addressed separately, but it has revealed that the
val_is_valid field is not initialized.

Fixes: 95573cac ("kconfig: cache expression values")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>