shell: roll back a change to the 'when' macro

Introduced in commit 1adc904ef3 from a week ago, but it turns out brcircle
has been consistently broken ever since.

shell/data.limg

@@ -5,8 +5,8 @@
     (do . [(mac do body `((fn () ,@body)))])
     (let . [(mac let (var val . body)
   `((fn (,var) ,@body) ,val))])
-    (when . [(mac when (cond . body)
-  `(if ,cond (do ,@body) ()))])
+    (when . [(mac when (cond body)
+  `(if ,cond ,body ()))])
     (hline1 . [(def hline1 (fn (screen y x xmax color)
   (while (< x xmax)
     (pixel screen x y color)