kconfig: clear expr::val_is_valid when allocated

Since commit 95573cac25 ("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: 95573cac25 ("kconfig: cache expression values")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kconfig/expr.c

@@ -47,6 +47,7 @@ static struct expr *expr_lookup(enum expr_type type, void *l, void *r)
 	e->type = type;
 	e->left._initdata = l;
 	e->right._initdata = r;
+	e->val_is_valid = false;
 
 	hash_add(expr_hashtable, &e->node, hash);