hush: do not assign to readonly VAR in "VAR=VAL CMD" syntax too

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/hush_test/hush-vars/readonly0.tests

@@ -1,10 +1,12 @@
+unset a b
+
 readonly a=A
 b=B
 readonly b
-# readonly on already readonly var is harmless
+# readonly on already readonly var is harmless:
 readonly b a
 readonly | grep '^readonly [ab]='
-# this should work
+# this should work:
 export a b
 export -n a b
 echo Ok:$?
@@ -12,13 +14,23 @@ env | grep -e^a= -e^b=  # shows nothing
 
 # these should all fail (despite the same value being assigned)
 # bash does not abort even in non-interactive more (in script)
-true
-a=A
+true; a=A
+echo Fail:$?
+true; readonly a=A
+echo Fail:$?
+
+# in bash, assignment in export fails, but export succeeds! :)
+# we don't mimic that!
+true; export a=Z
 echo Fail:$?; true
-readonly a=A
-echo Fail:$?; true
-export a=A
-echo Fail:$?; true
-a=A echo Visible:$? # command still runs
+#env | grep '^a='
+#echo "^^^a is exported"
+export -n a  # undo that bashism, if it happens
+
+export b
+# this fails to both set and export a:
+a=Z env | grep '^[ab]='  # command still runs
+echo "^^^a is not exported"
+
 unset a
 echo Fail:$?; true