Fix for longstanding bug where set -e would fail to erase elements from an array.

This was introduced in 7b3377e78c

tests/test3.in

@@ -127,3 +127,10 @@ else
 end
 
 
+set foo abc def
+set -e foo[1]
+if test $foo '=' def
+	echo Test 11 pass
+else
+	echo Test 11 fail
+end

tests/test3.out

@@ -8,3 +8,4 @@ Test 7 pass
 Test 8 pass
 Test 9 pass
 Test 10 pass
+Test 11 pass