hush: fix a corner case of empty "do \n done" structure

The structure is:

    while cmd; do
    done

bash doesn't accept it at all. We were accepting it but execution
was buggy.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/hush_test/hush-misc/while3.tests b/shell/hush_test/hush-misc/while3.tests
new file mode 100755
index 0000000..9132b5f
--- /dev/null
+++ b/shell/hush_test/hush-misc/while3.tests
@@ -0,0 +1,4 @@
+while false; do
+    # bash will require at least ":" here...
+done
+echo OK:$?