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

jv.c

@@ -230,9 +230,13 @@ static jv* jvp_array_read(jv a, int i) {
 }
 
 static jv* jvp_array_write(jv* a, int i) {
-  assert(i >= 0);
   jvp_array* array = jvp_array_ptr(*a);
 
+  if (i < 0)
+    i = array->length + i;
+  if (i < 0)
+    i = 0;
+
   int pos = i + jvp_array_offset(*a);
   if (pos < array->alloc_length && jvp_refcnt_unshared(a->u.ptr)) {
     // use existing array space

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
 #