.foo[-1] = ... trips assertion (fix #490)

tests/all.test

@@ -152,6 +152,26 @@ null
 [1,null,true,false,"abcdef",{},{"a":1,"b":2},[],[1,2,3,4,5],[1,2]]
 [null,"bc",[],[2,3],[2]]
 
+#
+# Negative array indices
+#
+
+.foo[-1] = 0
+null
+{"foo":[0]}
+
+.foo[-2] = 0
+null
+{"foo":[0]}
+
+.[-1] = 5
+[0,1,2]
+[0,1,5]
+
+.[-2] = 5
+[0,1,2]
+[0,5,2]
+
 #
 # Multiple outputs, iteration
 #