hush: fix SEGV in % expansion

function                                             old     new   delta
expand_variables                                    2203    2217     +14

diff --git a/shell/hush_test/hush-vars/var_posix1.right b/shell/hush_test/hush-vars/var_posix1.right
index 47d52a6..e6cba27 100644
--- a/shell/hush_test/hush-vars/var_posix1.right
+++ b/shell/hush_test/hush-vars/var_posix1.right
@@ -32,4 +32,5 @@
 ababcdcd
 Empty:
 ababcdcd}_tail
+ababcdcd
 end