hush: fix SEGV in % expansion

function                                             old     new   delta
expand_variables                                    2203    2217     +14

diff --git a/shell/hush_test/hush-arith/arith.right b/shell/hush_test/hush-arith/arith.right
index 8cde0ee..83155fb 100644
--- a/shell/hush_test/hush-arith/arith.right
+++ b/shell/hush_test/hush-arith/arith.right
@@ -63,9 +63,9 @@
 40 40
 hush: error in arithmetic
 hush: divide by 0
-hush: can't exec 'let': No such file or directory
+hush: can't execute 'let': No such file or directory
 hush: error in arithmetic
-hush: can't exec 'let': No such file or directory
+hush: can't execute 'let': No such file or directory
 abc
 def
 ghi
@@ -135,4 +135,4 @@
 42
 42
 42
-hush: can't exec 'a[b[c]d]=e': No such file or directory
+hush: can't execute 'a[b[c]d]=e': No such file or directory