core: check for negative indices in ResourceConfig.get

The bounds checking in ResourceConfig.get() was insufficient: it detected when the index was greater than or equal to cv.Len() but not when the index was less than zero. If the user provided an (invalid) configuration that referenced "foo.-1.bar", the provider would panic.

Now it behaves the same way as if the index were too high.

terraform/resource.go

@@ -346,7 +346,7 @@ func (c *ResourceConfig) get(
 				if err != nil {
 					return nil, false
 				}
-				if i >= int64(cv.Len()) {
+				if int(i) < 0 || int(i) >= cv.Len() {
 					return nil, false
 				}
 				current = cv.Index(int(i)).Interface()

terraform/resource_test.go

@@ -158,6 +158,14 @@ func TestResourceConfigGet(t *testing.T) {
 			Value: nil,
 		},
 
+		{
+			Config: map[string]interface{}{
+				"foo": []interface{}{1, 2, 5},
+			},
+			Key:   "foo.-1",
+			Value: nil,
+		},
+
 		// get from map
 		{
 			Config: map[string]interface{}{