Check that packets still exist when we resume

Possible nil pointer deference, after getting all the keys still in the
store it's possible that packets will arrive that cause them to be
removed. Now check that the packet returned from the
store is not nil

resolves #291

client.go

@@ -671,6 +671,9 @@ func (c *client) resume(subscription bool) {
 	storedKeys := c.persist.All()
 	for _, key := range storedKeys {
 		packet := c.persist.Get(key)
+		if packet == nil {
+			continue
+		}
 		details := packet.Details()
 		if isKeyOutbound(key) {
 			switch packet.(type) {