hush: fix 'eval ""' handling

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/ash_test/ash-misc/eval2.tests b/shell/ash_test/ash-misc/eval2.tests
new file mode 100755
index 0000000..6bfb87a
--- /dev/null
+++ b/shell/ash_test/ash-misc/eval2.tests
@@ -0,0 +1,4 @@
+false; eval; echo Zero:$?
+false; eval ""; echo Zero:$?
+false; eval "
+"; echo Zero:$?