federation: nil pointer deference when using remove read

```
level=error ts=2018-06-13T07:19:04.515149169Z caller=stdlib.go:89 component=web caller="http: panic serving [::1" msg="]:56202: runtime error: invalid memory address or nil pointer dereference"
level=error ts=2018-06-13T07:19:04.516199547Z caller=stdlib.go:89 component=web caller="http: panic serving [::1" msg="]:56204: runtime error: invalid memory address or nil pointer dereference"
level=error ts=2018-06-13T07:19:04.51717692Z caller=stdlib.go:89 component=web caller="http: panic serving [::1" msg="]:56206: runtime error: invalid memory address or nil pointer dereference"
level=error ts=2018-06-13T07:19:04.564952878Z caller=stdlib.go:89 component=web caller="http: panic serving [::1" msg="]:56208: runtime error: invalid memory address or nil pointer dereference"
level=error ts=2018-06-13T07:19:04.566575791Z caller=stdlib.go:89 component=web caller="http: panic serving [::1" msg="]:56210: runtime error: invalid memory address or nil pointer dereference"
level=error ts=2018-06-13T07:19:04.567106063Z caller=stdlib.go:89 component=web caller="http: panic serving [::1" msg="]:56212: runtime error: invalid memory address or nil pointer dereference"
```

When remove read is enabled, federation will call `q.Select(nil, mset...)`
which will break remote reads because it currently doesn't handle empty
SelectParams.

Signed-off-by: Corentin Chary <c.chary@criteo.com>

storage/remote/codec.go

@@ -91,10 +91,13 @@ func ToQuery(from, to int64, matchers []*labels.Matcher, p *storage.SelectParams
 	if err != nil {
 		return nil, err
 	}
+	var rp *prompb.ReadHints = nil
 
-	rp := &prompb.ReadHints{
+	if p != nil {
-		StepMs: p.Step,
+		rp = &prompb.ReadHints{
-		Func:   p.Func,
+			StepMs: p.Step,
+			Func:   p.Func,
+		}
 	}
 
 	return &prompb.Query{